fix(sanity): verify the ControlPath ssh RESOLVES, not the dir the image creates

recreate-sanity-check.sh asserted `/tmp/sshcm exists with mode 700` and printed
a green tick while every ssh in the container died rc=255. It was right about
what it checked: the breakage was in the directory a CONFIG named, not the one
the image creates, and the old check could not see the disagreement between them.

Found on the v1.9.2 first boot on emb-7kj4vr4g. A durable ~/.pi/ssh/config,
hand-written into the ~/.pi named volume by the previous session so it would
survive the recreate, declared `ControlPath /tmp/ssh-cm/%C` — with a hyphen.
Nothing here creates that path; the canonical spelling is /tmp/sshcm, identical
in Dockerfile.base, entrypoint-user.sh, recreate-sanity-check.sh and
smoke-test.sh. ControlMaster auto with an unusable ControlPath does not degrade
to an unmultiplexed connection — it fails hard:

    unix_listener: cannot bind to path /tmp/ssh-cm/<hash>: No such file or directory

rc=255, remote command never runs.

Now: resolve the ControlPath ssh itself would use via `ssh -G` and require its
parent to exist and be writable. -G applies real config precedence (first-value-
wins, the system drop-in, Include, an -F override), so it answers "which rule
captured this host" instead of re-implementing the guess, and it never opens a
connection — 0.116s for 48 hosts.

This also puts a check under a caveat documented in prose in Dockerfile.base
("SSH client defaults") and verified nowhere: a per-host ControlPath under a
read-only bind-mounted ~/.ssh gives the identical failure, "Read-only file
system". On the machine this was built on that is 16 of 50 hosts — freeipa-1..6,
gitea.egl.lan, runner-1..3, tor-ms22 — never reported by anything before.

Severity split is deliberate. Default ssh precedence legitimately lands in the
read-only ~/.ssh for any host whose own config pins it there, and the supported
workaround (ssh -F ~/.ssh-local/config, generated every start by
setup-lan-access.sh) already exists, so that is a warn. Failing it would paint
the script red on every run of every device, and a check that fires benignly
every time is one you learn to ignore — the same reasoning that keeps
lint-shell.sh at -S error. The sidecar route is prescribed, so there it is a
hard fail. Host lists cap at six names plus a count: unreadable output is
ignored output.

Teeth proven both directions, each sabotage confirmed by diff BEFORE the result
was believed: ControlPath -> nonexistent dir => rc=1; -> existing-but-read-only
dir => rc=1; restored => rc=0 with the sidecar byte-identical.

No config was added to fix the original problem — the fix was to DELETE
~/.pi/ssh/config, a third hand-maintained copy of what setup-lan-access.sh
already generates from version control on every container start, with fewer
features and one typo. The host-owned ~/.ssh/config is left alone on purpose:
those ~/.ssh/cm paths are correct on the host, where ~/.ssh is writable.

Also lint-shell.sh: the SC2088 count in the severity-choice rationale said 19
and is now 20, with the reproduce command recorded. That command needs a `$ `
prefix — a comment whose first word is "shellcheck" is parsed as a directive,
and the malformed one tripped SC1072/SC1073 at severity error. The gate caught
it on the very commit that introduced it.

scripts/recreate-sanity-check.sh

@@ -11,7 +11,8 @@
 #     pi-observational-memory / (studio variant) pi-studio package
 #     registrations in settings.json packages[]
 #   - Shell defaults re-seeded from /etc/skel-devbox
-#   - /tmp/sshcm exists with mode 700 (ssh ControlMaster dir)
+#   - ssh ControlMaster works: /tmp/sshcm exists 700 AND the ControlPath that
+#     ssh actually resolves (ssh -G) is a writable directory
 #   - /opt toolkits intact
 #   - Known expected-absences don't regress
 #
@@ -425,13 +426,115 @@ if [ -d /opt/pi-atelier ] && command -v jq >/dev/null 2>&1; then
 fi
 
 echo
-echo "-- ssh ControlMaster dir --"
+echo "-- ssh ControlMaster: socket dir + EFFECTIVE ControlPath --"
+# TWO LAYERS, and the second is the one that has actually broken in the field.
+#
+# LAYER 1 (original check): /tmp/sshcm, the directory entrypoint-user.sh creates
+# for the base image's system drop-in
+# (/etc/ssh/ssh_config.d/00-devbox-controlmaster.conf).
+#
+# LAYER 2 (added 2026-09-15): the directory a config NAMES — which is not the
+# same question, and asserting layer 1 is structurally blind to it. On
+# emb-7kj4vr4g a durable ~/.pi/ssh/config pointed ControlPath at /tmp/ssh-cm
+# (with a hyphen), a directory nothing in the image creates. EVERY ssh died
+#   unix_listener: cannot bind to path /tmp/ssh-cm/<hash>: No such file or directory
+# rc=255 with the remote command never running — while this script printed a
+# green tick for layer 1, truthfully, about the wrong object.
+#
+# The same rc=255 has a second, independent cause already documented in prose in
+# Dockerfile.base ("SSH client defaults" CAVEAT) and never verified anywhere: a
+# per-host `ControlPath ~/.ssh/cm/%r@%h:%p` inherited from a bind-mounted
+# READ-ONLY ~/.ssh. Measured to be the identical failure class:
+#   unix_listener: cannot bind to path ~/.ssh/cm/...: Read-only file system
+# So do not guess which config wins — ask ssh. `ssh -G` applies real config
+# precedence (first-obtained-value-wins, system drop-in, Include, -F override)
+# and prints the fully expanded ControlPath. Require its parent to exist and be
+# writable. Cost measured at 0.116 s for 48 hosts; -G never opens a connection.
 if [ -d /tmp/sshcm ] && [ "$(stat -c %a /tmp/sshcm 2>/dev/null)" = "700" ]; then
   pass "/tmp/sshcm exists with mode 700"
 else
   fail "/tmp/sshcm missing or not mode 700"
 fi
 
+# Probe one route. $1 = label, $2 = config to force with -F ("" = ssh's own
+# default precedence), $3 = severity when a ControlPath dir is unusable.
+#
+# SEVERITY SPLIT IS DELIBERATE. The default route legitimately resolves into the
+# read-only ~/.ssh on any host whose own config pins ControlPath there, and the
+# supported workaround (`ssh -F ~/.ssh-local/config`) already exists — so that
+# is a warn, not a fail. Failing it would paint this script red on every run of
+# every device, and a check that fires benignly every time is one you learn to
+# ignore. The sidecar route is the PRESCRIBED one, so there it is a hard fail.
+_ssh_cm_probe() {
+  local label="$1" cfg="${2:-}" sev="${3:-fail}"
+  local h out cm cp dir n=0 shown
+  local bad=()
+  while IFS= read -r h; do
+    [ -n "$h" ] || continue
+    if [ -n "$cfg" ]; then
+      out=$(ssh -F "$cfg" -G "$h" 2>/dev/null) || continue
+    else
+      out=$(ssh -G "$h" 2>/dev/null) || continue
+    fi
+    cm=$(printf '%s\n' "$out" | awk '/^controlmaster /{print $2; exit}')
+    case "$cm" in '' | no | none | false) continue ;; esac
+    cp=$(printf '%s\n' "$out" | awk '/^controlpath /{print $2; exit}')
+    case "$cp" in '' | none) continue ;; esac
+    n=$((n + 1))
+    dir=$(dirname "$cp")
+    if [ ! -d "$dir" ] || [ ! -w "$dir" ]; then
+      bad+=("$h")
+    fi
+  done <<< "$SSH_CM_HOSTS"
+
+  # Cap the host list. A 50-name line is the noisy gate this repo already warns
+  # about in lint-shell.sh: unreadable output is ignored output. Six names plus
+  # a count is enough to identify the class and act.
+  if [ "${#bad[@]}" -gt 0 ]; then
+    shown="${bad[*]:0:6}"
+    if [ "${#bad[@]}" -gt 6 ]; then
+      shown="$shown (+$(( ${#bad[@]} - 6 )) more)"
+    fi
+  fi
+
+  if [ "$n" -eq 0 ]; then
+    warn "$label: no host resolves to ControlMaster on — effective ControlPath not exercised"
+  elif [ "${#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 [$shown] — expected when ~/.ssh/config pins ControlPath inside the read-only ~/.ssh; use 'ssh -F ~/.ssh-local/config' (see Dockerfile.base CAVEAT)"
+  else
+    fail "$label: ${#bad[@]}/$n host(s) resolve ControlPath to a missing or unwritable dir [$shown] — ssh dies rc=255 'unix_listener: cannot bind to path' and the remote command never runs"
+  fi
+}
+
+if command -v ssh >/dev/null 2>&1; then
+  # Concrete Host aliases only: patterns (*, ?) and negations (!) are not
+  # connectable targets, so `ssh -G` on them proves nothing.
+  _cm_cfgs=()
+  if [ -r "$HOME/.ssh/config" ]; then _cm_cfgs+=("$HOME/.ssh/config"); fi
+  if [ -r "$HOME/.ssh-local/config" ]; then _cm_cfgs+=("$HOME/.ssh-local/config"); fi
+  if [ "${#_cm_cfgs[@]}" -gt 0 ]; then
+    SSH_CM_HOSTS=$(awk 'tolower($1)=="host"{for(i=2;i<=NF;i++) if ($i !~ /[*?!]/) print $i}' \
+      "${_cm_cfgs[@]}" 2>/dev/null | sort -u)
+  else
+    SSH_CM_HOSTS=""
+  fi
+
+  if [ -z "$SSH_CM_HOSTS" ]; then
+    warn "no concrete Host aliases in ~/.ssh/config or ~/.ssh-local/config — effective ControlPath not verified"
+  else
+    _ssh_cm_probe "default ssh precedence" "" warn
+    if [ -r "$HOME/.ssh-local/config" ]; then
+      _ssh_cm_probe "ssh -F ~/.ssh-local/config" "$HOME/.ssh-local/config" fail
+    else
+      warn "~/.ssh-local/config absent — setup-lan-access.sh did not run; the prescribed multiplex route is unverified"
+    fi
+  fi
+else
+  warn "ssh not on PATH — effective ControlPath not verified"
+fi
+
 echo
 echo "-- Shell defaults re-seeded from /etc/skel-devbox --"
 if [ -f "$HOME/.bash_aliases" ]; then