feat: bake shellcheck and add a client-side pre-push lint gate

v1.8.14 made shell lint a RELEASE gate (scripts/lint-shell.sh, shared by lint.yml
and the new lint-gate job that resolve-versions depends on), and that script
correctly exits 2 when shellcheck is absent -- "a gate that cannot run must not
pass". Measured on v1.8.14 on 2026-09-09 by three routes (command -v, dpkg -l, a
filesystem search): shellcheck was NOT IN THE IMAGE AT ALL. So the gate could not
be run by a developer in any container, only in CI, and the loop stayed
write-shell -> push -> wait for CI -> discover. That is the loop the gate was
added to shorten, after v1.8.14's first attempt burned ~46 min on a tree whose
lint had already been red for 24 hours.

shellcheck 0.10.0-1 added to the Dockerfile.base apt block: ~39 MB installed
(Installed-Size 40112 KB), measured to pull ZERO additional packages under
--no-install-recommends because libc6/libffi8/libgmp10 are already present.
NOTE this forces one full base rebuild -- base-decide hashes Dockerfile.base +
rootfs/, so unlike a scripts/ change it cannot reuse the existing base- layer.

hooks/pre-push is opt-in per clone (git config core.hooksPath hooks), bypassable
with --no-verify, and execs scripts/lint-shell.sh rather than reimplementing it
-- one copy, because a duplicated check that drifts is the failure this repo
keeps paying for. Matches the idiom skillset/ and myconfigs/ already use.

WHY THIS REPO HAD NO HOOKS, since it was reported as drift and is not: a peer
asked tor-ms22 for core.hooksPath per clone on the premise that unset meant the
gates were unverified there. Measured: pi-devbox unset, skillset hooks, myconfigs
common/hooks, pi-toolkit unset -- but `git ls-files | grep -i hook` is EMPTY in
both pi-devbox and pi-toolkit, so there was nothing to point at on any machine
and unset was the only correct value. This closes the real half for pi-devbox;
pi-toolkit still ships none.

Verified, expected result written down before each check:
  * refusal paths -- shellcheck absent => rc 2 with the remedy named; linter
    missing => rc 2. Never waved through on the assumption CI will catch it.
  * the hook is IN the scan set -- "Checking 14 shell file(s)" with it present,
    13 with it moved aside, so the extensionless file is found by the shebang
    half of the linter's two-signal union. This check exists because the first
    attempt was ambiguous: a planted `[ $UNSET_VAR = "x" ]` was not reported,
    which could equally have meant "not scanned" or "below -S error". It was the
    latter. A count that moves is unambiguous; a clean run is not.
  * it catches the REAL v1.8.14 defect -- planting `echo 'the fleet\'s thing'`
    in hooks/pre-push yields SC1073/SC1072 at severity error, rc=1.

And the gate earned its keep inside this commit: the first version of the
smoke-test assertion carried a comment beginning "# shellcheck is a GATE
DEPENDENCY", and a comment whose first word is the tool's name is parsed as a
DIRECTIVE, not a comment. The new gate failed it with SC1073/SC1072 before the
push -- same family as the v1.8.14 apostrophe, a line that reads as prose to a
human and as syntax to the parser.

scripts/smoke-test.sh

@@ -105,6 +105,18 @@ else
   run "node"            "node --version"
 fi
 run "git"             "git --version"
+# NOTE: the shellcheck binary is a GATE DEPENDENCY, not a convenience.
+# scripts/lint-shell.sh is the release gate (the lint-gate job resolve-versions
+# depends on) and it exits 2 when the binary is missing, by design — "a gate that
+# cannot run must not pass". Measured on v1.8.14: it was absent from the image, so
+# that gate could not be run by a developer in ANY container, only in CI.
+# Asserted here so its absence fails a build instead of being discovered by a hook
+# that then refuses every push (hooks/pre-push).
+#
+# This comment must not BEGIN with the tool's name: a line starting with
+# `# shellcheck` is parsed as a DIRECTIVE, not a comment (SC1073/SC1072). The
+# gate added in this same change caught that here, before the push.
+run "shellcheck (lint gate dependency)" "shellcheck --version | grep -qE '^version: [0-9]'"
 run "aws"             "aws --version"
 run "uv"              "uv --version"
 run "nvim"            "nvim --version"