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/actions/check-clean-worktree/action.yml

@@ -0,0 +1,19 @@
+name: check-clean-worktree
+description: Fail when a CI job leaves tracked changes or untracked files in the repository worktree.
+
+runs:
+  using: composite
+  steps:
+    - name: Check for a clean worktree
+      shell: bash
+      run: |
+        set -euo pipefail
+
+        status="$(git -C "${GITHUB_WORKSPACE}" status --porcelain=v1 --untracked-files=normal --ignore-submodules=none)"
+        if [[ -z "${status}" ]]; then
+          exit 0
+        fi
+
+        echo "::error::CI job left tracked changes or untracked files in the repository worktree"
+        printf '%s\n' "${status}"
+        exit 1

.github/actions/setup-bazel-ci/action.yml

@@ -34,6 +34,10 @@ runs:
 
     - name: Set up Bazel
       uses: bazel-contrib/setup-bazel@c5acdfb288317d0b5c0bbd7a396a3dc868bb0f86 # 0.19.0
+      # Without an explicit Bazelisk version, setup-bazel leaves PATH unchanged
+      # and Windows can use the runner's standalone Bazel, ignoring .bazelversion.
+      with:
+        bazelisk-version: 1.28.1
 
     - name: Configure Bazel repository cache
       id: configure_bazel_repository_cache