ci: pin actions to commit SHAs and add shellcheck

.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

CONTRIBUTING.md

@@ -113,6 +113,14 @@ uv pip install -e ".[test]"
 > `specify_cli` to this checkout's `src/`. This matches the gotcha documented in
 > `AGENTS.md` (Common Pitfalls).
 
+#### Shell scripts
+
+```bash
+git ls-files -z -- '*.sh' | xargs -0 shellcheck --severity=error
+```
+
+The CI `lint.yml` `shellcheck` job blocks at `--severity=error` to catch real bugs while leaving stylistic warnings (SC2155 etc.) advisory.
+
 ### Manual testing
 
 #### Testing setup

tests/test_github_workflows.py

@@ -0,0 +1,34 @@
+"""Static checks for repository GitHub Actions workflows."""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+WORKFLOWS_DIR = REPO_ROOT / ".github" / "workflows"
+# Match both the dedicated-step form (`        uses: x@sha`) and the
+# inline shorthand (`      - uses: x@sha`) used in catalog-assign.yml.
+USES_RE = re.compile(r"^\s*(?:-\s*)?uses:\s*(?P<ref>\S+)", re.MULTILINE)
+
+
+def test_github_actions_are_pinned_to_full_commit_shas():
+    unpinned_refs = []
+
+    workflows = sorted(
+        list(WORKFLOWS_DIR.glob("*.yml")) + list(WORKFLOWS_DIR.glob("*.yaml"))
+    )
+    assert workflows
+
+    for workflow in workflows:
+        workflow_text = workflow.read_text(encoding="utf-8")
+        for match in USES_RE.finditer(workflow_text):
+            uses_ref = match.group("ref")
+            if uses_ref.startswith(("./", "../")):
+                continue
+            if re.search(r"@[0-9a-f]{40}$", uses_ref):
+                continue
+            unpinned_refs.append(f"{workflow.relative_to(REPO_ROOT)}: {uses_ref}")
+
+    assert unpinned_refs == []