lint: shellcheck the repo's own shell scripts, not just workflow run: steps

lint.yml has shellchecked every workflow `run:` step since the dash-vs-bash
incidents, but nothing had ever pointed shellcheck at entrypoint.sh, scripts/*.sh
or the extensionless tools under rootfs/usr/local/bin/. That gap is not
hypothetical: the skillset repo's ci-release-watcher template shipped
`echo "$json" | python3 <<'EOF' ... json.load(sys.stdin)` for two months, where
the heredoc IS python's stdin (no script arg) so the load hit EOF and the
function silently returned nothing. shellcheck names exactly that at severity
ERROR — SC2259, "This redirection overrides piped input" — and could have named
it the whole time.

New step in the existing actionlint job, so no second container pull: shellcheck
-S error plus bash -n over every shell file, discovered as *.sh UNION a shebang
scan (the glob alone misses pi-devbox-version, devbox-skill-reconcile, dot-watch
and studio-expose; a shebang scan alone would miss a sourced fragment without
one). Fails loudly on a zero-file match, because a green tick over an empty set
is not a check.

Severity chosen by measurement, not taste: -S error is 0 findings across all 11
shell files today, so the gate is green on arrival with no cleanup, while
-S warning is NOT free (19x SC2088 tilde-in-quotes in recreate-sanity-check.sh
plus assorted SC2016, all intentional) and would train everyone to ignore the
job — the same reasoning as the SHELLCHECK_OPTS exclusions already on the
actionlint step.

.gitea/workflows/lint.yml

@@ -52,6 +52,55 @@ jobs:
           apt-get update
           apt-get install -y --no-install-recommends shellcheck python3-yaml
 
+      - name: "Shellcheck + syntax-check repository scripts (severity: error)"
+        # Gap being closed: everything else in this job shellchecks workflow
+        # `run:` steps ONLY, via actionlint. The repo's own shell scripts —
+        # entrypoint.sh, scripts/*.sh, and the extensionless tools under
+        # rootfs/usr/local/bin/ — have never been shellchecked. That exact gap
+        # (a sibling repo with no shell-script lint at all) is how a defect
+        # shipped invisibly for two months: `echo "$json" | python3 <<'EOF'
+        # ... json.load(sys.stdin)` cannot work — with no script argument
+        # python reads its SCRIPT from stdin, so the heredoc IS stdin and the
+        # json.load call hits EOF. shellcheck flags exactly this at severity
+        # ERROR (SC2259, "This redirection overrides piped input"); nothing
+        # ever ran it. Measured before adding this gate: `-S error` is 0
+        # findings across every shell file in THIS repo today, so it is free
+        # to add. `-S warning` is NOT free here (19x SC2088 tilde-in-quotes in
+        # scripts/recreate-sanity-check.sh, plus assorted SC2016 — both
+        # intentional), so warning-level would train people to ignore the job;
+        # hence error-only, matching the SHELLCHECK_OPTS philosophy below.
+        #
+        # Discovery is *.sh UNION a shebang scan, because rootfs/usr/local/
+        # bin/{pi-devbox-version,devbox-skill-reconcile,dot-watch,studio-expose}
+        # are shell scripts with no extension. -print0/mapfile -d '' so a path
+        # with a space cannot silently split, and the file count is asserted
+        # non-zero — a green tick over an empty file set is not a check.
+        run: |
+          # Union of two signals, because either alone misses a real case:
+          # a shebang scan misses a sourced fragment with no shebang, and a
+          # *.sh glob misses the extensionless tools in rootfs/usr/local/bin/.
+          # Silent skipping is precisely the failure mode this gate exists to
+          # prevent, so err toward over-collecting.
+          mapfile -d '' -t all_files < <(find . -not -path './.git/*' -type f -print0)
+          sh_files=()
+          for f in "${all_files[@]}"; do
+            case "$f" in *.sh) sh_files+=("$f"); continue;; esac
+            if head -n1 "$f" 2>/dev/null | grep -qE '^#!.*\b(bash|sh)\b'; then
+              sh_files+=("$f")
+            fi
+          done
+          echo "Checking ${#sh_files[@]} shell file(s)"
+          if [ "${#sh_files[@]}" -eq 0 ]; then
+            echo "::error::no shell files found — the shebang scan or the checkout is wrong"
+            exit 1
+          fi
+          shellcheck -S error -f gcc "${sh_files[@]}"
+          rc=0
+          for f in "${sh_files[@]}"; do
+            bash -n "$f" || { echo "::error file=$f::bash -n failed"; rc=1; }
+          done
+          exit "$rc"
+
       - name: Gitea shell guard (catches the actionlint blind spot)
         # actionlint models GitHub Actions, where the default run shell is
         # bash, so it does NOT flag bash syntax in a step that merely OMITS