ci: gate the release on shell lint, from one shared script

v1.8.14's first attempt spent ~46 minutes building a base image for a tree whose
own lint had been failing for 24 hours. shellcheck had already flagged the
defect (SC2289, severity error) on the push that introduced it; the lint
workflow went red at run 186 and nobody read it.

lint.yml deliberately skips tag pushes and its reasoning is sound -- the tagged
tree was already linted on main, and a tag-ref lint run sorts above the publish
run, making a release look finished before anything ships. The missing invariant
was never "lint the tag". It was "do not RELEASE a tree whose lint failed", and
only a job inside the publish workflow can enforce that.

So: extract the shell-lint logic from lint.yml into scripts/lint-shell.sh and
call it from both places, then add a lint-gate job that resolve-versions depends
on. resolve-versions is the graph root, so gating it gates everything. Cost is
~40 s at the front of a release; the alternative already cost fifty minutes.

Extracted rather than copied on purpose. A second copy of a check is the drift
this repo keeps paying for -- the same evening produced a skillset mirror that
had sat 9579 B behind its upstream through two consecutive edits.

The script adds one behaviour the inline version lacked: if shellcheck is not
installed it exits 2 rather than silently finding nothing, inheriting the
existing "a gate that cannot run must not pass" rule from hooks/pre-commit in
the skillset repo. Without that, reordering the install step away would turn the
gate into a green tick over zero checks.

Verified locally with a stubbed shellcheck (the real binary is not in the
devbox), five cases, each with its expectation stated first: absent shellcheck
-> rc=2; stub pass -> rc=0 and a non-zero file count; stub fail -> rc=1; a
deliberately unterminated `if` planted in scripts/ -> rc=1 via the bash -n half,
naming the file; removal -> rc=0 again. Discovery cross-checks against CI's own
number: the inline version reported 12 files, the extracted one reports 13, the
difference being lint-shell.sh itself. YAML re-parsed (10 jobs, was 9) with an
assertion that resolve-versions needs lint-gate, and the repo's
check-workflow-shell.sh guard still passes.

.gitea/workflows/docker-publish.yml

@@ -157,7 +157,41 @@ jobs:
   # buildcache silently reuses the layer from whatever pi version was
   # current when the cache was first populated. Same class of bug as
   # pi-devbox v0.74.0..v0.75.5 (fixed in v0.75.5b 2026-05-23).
+  # ── release gate ──────────────────────────────────────────────
+  # Refuse to spend a base build on a tree whose own shell scripts do not lint.
+  #
+  # v1.8.14's first attempt is why this exists. smoke and smoke-studio both failed
+  # at scripts/smoke-test.sh:770 AFTER build-base had already spent ~46 minutes,
+  # on a defect shellcheck had flagged as SC2289 (severity error) a day earlier:
+  # the lint workflow went red on the very push that introduced it (run 186) and
+  # stayed red for runs 187 and 188, unread.
+  #
+  # lint.yml deliberately does not run on tag pushes, and its reasoning is sound
+  # (the tagged tree was already linted on main; a tag-ref lint run sorts above
+  # the publish run and makes a release look finished before anything ships). The
+  # missing invariant was never "lint the tag" -- it was "do not RELEASE a tree
+  # whose lint failed", and only a job inside THIS workflow can enforce that.
+  #
+  # ~40 s, ahead of everything expensive, and it runs scripts/lint-shell.sh --
+  # the same file lint.yml calls, not a second copy that drifts.
+  lint-gate:
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install shellcheck
+        run: |
+          apt-get update
+          apt-get install -y --no-install-recommends shellcheck
+
+      - name: "Shellcheck + syntax-check repository scripts (severity: error)"
+        run: bash scripts/lint-shell.sh
+
   resolve-versions:
+    # Gated: a defective tree must not reach a 46-minute base build.
+    needs: [lint-gate]
     runs-on: ubuntu-latest
     container:
       image: catthehacker/ubuntu:act-latest