ci: fail jobs that dirty the worktree (#29720)

## Why

CI jobs should not silently leave tracked changes or untracked files in
the repository worktree.

## What

- Add a shared final worktree-cleanliness action to 19 checkout-bearing
PR and main CI jobs.
- Ignore the intentional SDK scratch directory and nested V8 checkout.
- Pin Bazelisk in shared CI setup so `.bazelversion` remains
authoritative, avoiding `MODULE.bazel.lock` deltas on Windows runners.
- Leave `rust-ci-full` and release-only workflows unchanged.
- Update `AGENTS.md` to discourage review bots from asking for
`MODULE.bazel.lock` changes.

.github/workflows/codespell.yml

@@ -28,3 +28,7 @@ jobs:
         uses: codespell-project/actions-codespell@8f01853be192eb0f849a5c7d721450e7a467c579 # v2.2
         with:
           ignore_words_file: .codespellignore
+
+      - name: Check for a clean worktree
+        if: always() && !cancelled()
+        uses: ./.github/actions/check-clean-worktree