ci: pin actions to commit SHAs and add shellcheck (#3126)

* ci: pin actions to commit SHAs and add shellcheck

Pin actions/github-script in catalog-assign.yml to a full commit SHA; all
other workflows were already pinned. Add a repo-wide regression test that
every workflow `uses:` ref is pinned to a 40-char commit SHA.

Add a shellcheck job to lint.yml (--severity=error over scripts/bash/*.sh)
and document the local command in CONTRIBUTING.md.

* ci: use repo-standard actions/checkout v7.0.0 in shellcheck job

* ci: shellcheck all tracked shell scripts

Assisted-by: Codex (model: GPT-5, autonomous)

* ci: address workflow hygiene review feedback

Assisted-by: Codex (model: GPT-5, autonomous)

.github/workflows/catalog-assign.yml

@@ -19,7 +19,7 @@ jobs:
     permissions:
       issues: write
     steps:
-      - uses: actions/github-script@v9
+      - uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
         with:
           script: |
             const issue = context.payload.issue;

.github/workflows/lint.yml

@@ -42,3 +42,15 @@ jobs:
           globs: |
             '**/*.md'
             !extensions/**/*.md
+
+  shellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+
+      # shellcheck is preinstalled on ubuntu-latest runners.
+      # Start at --severity=error to block real bugs without flagging style
+      # (notably SC2155). Tighten in a follow-up after cleanup.
+      - name: Run shellcheck on shell scripts
+        run: git ls-files -z -- '*.sh' | xargs -0 shellcheck --severity=error