fix(ssh): wire git core.sshCommand to the writable sidecar, and assert it

~/.ssh is commonly bind-mounted READ-ONLY from the host, so a per-host
`ControlPath ~/.ssh/cm/%r@%h:%p` — the standard CGNAT multiplexing recipe, and
correct on the host — resolves inside an unwritable dir in the container. Every
push dies `unix_listener: cannot bind to path ...: Read-only file system`,
behind git's misleading "make sure you have the correct access rights".

setup-lan-access.sh already writes the fix: ~/.ssh-local/config overrides
ControlPath into the writable ~/.ssh-local/cm BEFORE `Include ~/.ssh/config`,
so -F repairs the socket path and keeps every per-host User/Port/IdentityFile.
entrypoint-user.sh now points git at it, guarded on the sidecar existing —
setup-lan-access.sh writes none on native Linux Docker, where -F at a missing
file would break every git-over-ssh call instead of fixing one. An existing
core.sshCommand is left alone (first-wins, as for the three git settings above).

Why code and not another doc line: the remedy was already in the global
AGENTS.md, in pi-devbox-environment SKILL.md §3, in 24 MemPalace drawers from
three devices, and printed verbatim by recreate-sanity-check.sh — and an agent
that had run that script two hours earlier still hit the failure and reinvented
a /tmp/sshcm workaround. A fifth copy was not the missing piece.

Assertions, each where it can actually pass:
- smoke-test.sh: two STATIC greps (wiring line + its [ -r ] guard). `run` uses
  --entrypoint="", so asserting the runtime value there would repeat the v1.8.0
  mistake of an assertion that cannot pass, unvalidated until the next tag.
- smoke-test.sh runtime phase: a BICONDITIONAL — sidecar present => must route
  through it; absent => must be unset. The absent arm is the one CI exercises
  (native Linux runner), so "is set" would have failed CI for a correct image.
- recreate-sanity-check.sh: the runtime assertion, plus an explicit fail for the
  inverted state (set while the sidecar is missing). The permanent "default ssh
  precedence" warning keeps its severity but now states that it is structural and
  can never reach zero, and whether git is wired, unwired, or has no sidecar.

All five arms exercised against the real script before commit; that caught a
defect in the first draft, which reported "git IS wired ... unaffected" about a
state where the sidecar was gone and every git-over-ssh call failed.

Host ~/.ssh/config needs no change: the same line is right on the host and
unusable through a read-only mount, so the fix belongs in the container layer.

scripts/recreate-sanity-check.sh

@@ -502,13 +502,33 @@ _ssh_cm_probe() {
   elif [ "${#bad[@]}" -eq 0 ]; then
     pass "$label: $n ControlMaster host(s), every ControlPath dir exists and is writable"
   elif [ "$sev" = "warn" ]; then
-    warn "$label: ${#bad[@]}/$n host(s) resolve ControlPath to a missing or unwritable dir [$shown] — expected when ~/.ssh/config pins ControlPath inside the read-only ~/.ssh; use 'ssh -F ~/.ssh-local/config' (see Dockerfile.base CAVEAT)"
+    warn "$label: ${#bad[@]}/$n host(s) resolve ControlPath to a missing or unwritable dir [$shown] — STRUCTURAL and permanent while ~/.ssh/config pins ControlPath inside the read-only ~/.ssh, so this line can never reach zero and is not a to-do; $_git_ssh_note (see Dockerfile.base CAVEAT)"
   else
     fail "$label: ${#bad[@]}/$n host(s) resolve ControlPath to a missing or unwritable dir [$shown] — ssh dies rc=255 'unix_listener: cannot bind to path' and the remote command never runs"
   fi
 }
 
 if command -v ssh >/dev/null 2>&1; then
+  # Whether git-over-ssh already routes through the sidecar decides how much the
+  # permanent default-route warning below actually matters, so state it IN that
+  # message rather than leaving each reader to work it out. Asserted properly as
+  # its own pass/fail in the next section.
+  #
+  # THREE states, not two, and the [ -r ] test is why: core.sshCommand NAMING the
+  # sidecar does not mean the sidecar EXISTS. Without that test, the arm where the
+  # path is wired but the file is gone printed "git IS wired ... unaffected" about
+  # a state in which every single git-over-ssh call fails. Found by exercising all
+  # five arms of this check rather than only the healthy one.
+  if [ ! -r "$HOME/.ssh-local/config" ]; then
+    _git_ssh_note="there is no ssh sidecar on this host, so nothing for -F to point at and these hosts cannot multiplex at all — see the git-over-ssh check below"
+  elif command -v git >/dev/null 2>&1 &&
+       git config --global --get core.sshCommand 2>/dev/null |
+         grep -qF -- "-F $HOME/.ssh-local/config"; then
+    _git_ssh_note="git IS wired to the sidecar (core.sshCommand), so git push/fetch is unaffected; bare 'ssh' to these hosts still needs 'ssh -F ~/.ssh-local/config'"
+  else
+    _git_ssh_note="git is NOT wired to the sidecar (see the git-over-ssh check below), so both git and bare 'ssh' need 'ssh -F ~/.ssh-local/config'"
+  fi
+
   # Concrete Host aliases only: patterns (*, ?) and negations (!) are not
   # connectable targets, so `ssh -G` on them proves nothing.
   _cm_cfgs=()
@@ -535,6 +555,46 @@ else
   warn "ssh not on PATH — effective ControlPath not verified"
 fi
 
+echo
+echo "-- git-over-ssh routed through the writable ssh sidecar --"
+# The one question in this area that is BINARY, FIXABLE, and therefore worth a
+# check that can reach zero and stay there.
+#
+# The ControlPath warning above cannot: while a bind-mounted ~/.ssh/config pins
+# ControlPath inside the read-only ~/.ssh, the default route will ALWAYS resolve
+# to an unwritable dir, so that line fires benignly on every run of every device
+# forever — and the script's own comment above says why that is dangerous: it is
+# a warning you learn to skip. MEASURED 2026-09-22, exactly that: an agent ran
+# this script, recorded "two by-design warnings", then two hours later hit
+# `unix_listener: cannot bind ... Read-only file system` on `git push`, failed to
+# connect it to the warning it had already read, and reinvented a /tmp/sshcm
+# workaround — while the remedy string sat inside the dismissed warning.
+# entrypoint-user.sh now wires core.sshCommand to the sidecar so nobody has to
+# know; this section asserts the wiring actually happened, which is the part a
+# reader can act on.
+if ! command -v git >/dev/null 2>&1; then
+  warn "git not on PATH — sidecar wiring not verified"
+elif [ ! -r "$HOME/.ssh-local/config" ]; then
+  # No sidecar (native Linux Docker, where setup-lan-access.sh writes none). The
+  # correct state is UNSET: -F pointing at a missing file breaks every
+  # git-over-ssh call, which is worse than the problem being solved.
+  if [ -z "$(git config --global --get core.sshCommand 2>/dev/null || true)" ]; then
+    pass "no ssh sidecar on this host and core.sshCommand correctly unset (guard holds)"
+  else
+    fail "core.sshCommand is set but ~/.ssh-local/config does not exist — every git-over-ssh call dies on a missing -F file; entrypoint-user.sh's [ -r ] guard did not hold"
+  fi
+else
+  _git_ssh_cmd=$(git config --global --get core.sshCommand 2>/dev/null || true)
+  case "$_git_ssh_cmd" in
+    *"-F $HOME/.ssh-local/config"*)
+      pass "git core.sshCommand routes through the sidecar ($_git_ssh_cmd)" ;;
+    '')
+      fail "a sidecar exists but git core.sshCommand is unset — 'git push' to any host whose config pins ControlPath inside the read-only ~/.ssh dies rc=255 behind git's misleading 'correct access rights'. entrypoint-user.sh should set it; expected on images built before that wiring landed, where the fix is: git config --global core.sshCommand \"ssh -F \$HOME/.ssh-local/config\"" ;;
+    *)
+      warn "git core.sshCommand set to something else and left alone (first-wins, deliberate): $_git_ssh_cmd" ;;
+  esac
+fi
+
 echo
 echo "-- Shell defaults re-seeded from /etc/skel-devbox --"
 if [ -f "$HOME/.bash_aliases" ]; then

scripts/smoke-test.sh

@@ -250,6 +250,22 @@ run_expect "remote-palace-without-inbox skip is announced, not silent" \
     "MemPalace catch-up skipped"
 run "...and the skip notice names the variable that fixes it" \
     "grep -A6 'MemPalace catch-up skipped' /usr/local/bin/entrypoint-user.sh | grep -q 'MEMPALACE_PI_SSH_TARGET'"
+# git-over-ssh must be wired to the writable ssh sidecar, because ~/.ssh is
+# commonly bind-mounted READ-ONLY and a per-host `ControlPath ~/.ssh/cm/...`
+# inherited from it kills every push with `unix_listener: cannot bind ...:
+# Read-only file system` behind git's misleading "correct access rights".
+#
+# STATIC assertion, deliberately. `run` executes `docker run --entrypoint=""`, so
+# entrypoint-user.sh never runs here and `git config --global core.sshCommand` is
+# necessarily unset — asserting the VALUE in this harness would repeat the v1.8.0
+# mistake (an assertion that cannot pass, unvalidated until the next tag). What
+# IS checkable at build time is that the wiring code shipped in the image. The
+# runtime value is asserted below in the Runtime deployment section, where the
+# real entrypoint chain has run.
+run "entrypoint wires git core.sshCommand to the ssh sidecar" \
+    "grep -q 'core.sshCommand \"ssh -F' /usr/local/bin/entrypoint-user.sh"
+run "...and guards it on the sidecar existing (native Linux has none)" \
+    "grep -B2 'core.sshCommand \"ssh -F' /usr/local/bin/entrypoint-user.sh | grep -q '\\[ -r \"\$HOME/.ssh-local/config\" \\]'"
 # A remote mine that FAILS must not report success. MCP answers a hard tool
 # failure with HTTP 200 and the tool's own JSON escaped inside
 # result.content[].text, so the feeder's old `'\"error\"' in body` check could
@@ -659,6 +675,23 @@ exec_test "settings.json bootstrapped"             'test -f $HOME/.pi/agent/sett
 exec_test "pi-devbox-environment skill linked"     'test -L $HOME/.agents/skills/pi-devbox-environment && test -f $HOME/.agents/skills/pi-devbox-environment/SKILL.md && echo ok'
 exec_test "pi-extensions skill linked (fallback)"  'test -L $HOME/.agents/skills/pi-extensions && test -f $HOME/.agents/skills/pi-extensions/SKILL.md && echo ok'
 exec_test "mempalace skill linked (fallback)"      'test -L $HOME/.agents/skills/mempalace && test -f $HOME/.agents/skills/mempalace/SKILL.md && echo ok'
+# git-over-ssh sidecar wiring, asserted as a BICONDITIONAL rather than "is set".
+# setup-lan-access.sh writes ~/.ssh-local/config only on VM-backed hosts, and a
+# CI runner is native Linux Docker — so "core.sshCommand is set" would fail here
+# for a correct image, which is precisely the v1.8.0 trap (an assertion whose
+# environment was never checked, unvalidated until the next tag). Both arms are
+# real: sidecar present => must route through it; sidecar absent => must be UNSET,
+# because -F pointing at a missing file breaks every git-over-ssh call and is
+# worse than the problem being fixed. This arm is the one CI actually exercises,
+# so CI validates the guard; the other is covered by the static greps above and
+# by scripts/recreate-sanity-check.sh on a real device.
+exec_test "git core.sshCommand matches sidecar presence" '
+  cmd=$(git config --global --get core.sshCommand 2>/dev/null || true)
+  if [ -r "$HOME/.ssh-local/config" ]; then
+    case "$cmd" in *"-F $HOME/.ssh-local/config"*) echo "wired: $cmd" ;; *) exit 1 ;; esac
+  else
+    [ -z "$cmd" ] || exit 1; echo "no sidecar, correctly unset"
+  fi'
 # The vendored mempalace snapshot is refreshed MANUALLY per release (see
 # rootfs/usr/local/share/pi-devbox/skills/VENDORED.md). Through v1.8.4 it also
 # silently SHADOWED the live skillset copy, so staleness was invisible — and the