chore: pin actions to commit SHAs and add Skills section to CLAUDE.md

Pin all GitHub Actions to commit SHAs instead of mutable version tags
across ci.yml, release.yml, maintenance.yml, and all reusable workflows.
This prevents supply-chain attacks via tag hijacking.

Add the required Skills section to CLAUDE.md mapping project files
(README.md, .github/workflows/*.yml) to their respective review skills.

CLAUDE.md

@@ -59,3 +59,14 @@ Follow the formats in CONTRIBUTING.md:
 - Hooks: JSON with matcher and hooks array
 
 File naming: lowercase with hyphens (e.g., `python-reviewer.md`, `tdd-workflow.md`)
+
+## Skills
+
+Use the following skills when working on related files:
+
+| File(s) | Skill |
+|---------|-------|
+| `README.md` | `/readme` |
+| `.github/workflows/*.yml` | `/ci-workflow` |
+
+When spawning subagents, always pass conventions from the respective skill into the agent's prompt.