feat(maintenance): add post-checkout hook to auto-run pnpm install on dependency changes (#13033)

### What this PR does

Before this PR:
After switching branches with `git checkout`, dependencies are not
automatically synced, potentially causing stale `node_modules`.

After this PR:
A prek `post-checkout` hook automatically runs `pnpm install` when
`package.json` or `pnpm-lock.yaml` changes between branches. If no
dependency files changed, the install is skipped.

Fixes # N/A

### Why we need it and why it was done in this way

The following tradeoffs were made:
Uses an inline bash script in the prek hook entry to keep configuration
self-contained without requiring an external script file.

The following alternatives were considered:
A raw `.git/hooks/post-checkout` script was considered, but using prek
keeps all hooks managed in one place via `.pre-commit-config.yaml`.

Links to places where the discussion took place: N/A

### Breaking changes

None.

### Special notes for your reviewer

- The hook uses `PRE_COMMIT_FROM_REF` and `PRE_COMMIT_TO_REF`
environment variables provided by prek during `post-checkout` stage.
- Requires `prek install -t post-checkout` to register the hook after
pulling this change.

### Checklist

- [x] PR: The PR description is expressive enough and will help future
contributors
- [x] Code: Write code that humans can understand and Keep it simple
- [x] Refactor: You have left the code cleaner than you found it (Boy
Scout Rule)
- [ ] Upgrade: Impact of this change on upgrade flows was considered and
addressed if required
- [ ] Documentation: N/A

### Release note

```release-note
NONE
```

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.pre-commit-config.yaml

@@ -1,3 +1,5 @@
+default_install_hook_types: [pre-commit, post-checkout]
+
 repos:
   - repo: local
     hooks:
@@ -20,4 +22,12 @@ repos:
         language: system
         entry: pnpm biome format --write --no-errors-on-unmatched
         files: '\.(json|yml|yaml|css|html)$'
-        pass_filenames: true
+        pass_filenames: true
+
+      - id: post-checkout-install
+        name: pnpm install after checkout
+        language: system
+        entry: bash -c 'if [ "$PRE_COMMIT_FROM_REF" != "$PRE_COMMIT_TO_REF" ] && git diff --name-only "$PRE_COMMIT_FROM_REF" "$PRE_COMMIT_TO_REF" -- "**/package.json" pnpm-lock.yaml | grep -q .; then echo "Dependencies changed, running pnpm install..."; pnpm install; else echo "No dependency changes, skipping pnpm install."; fi'
+        stages: [post-checkout]
+        always_run: true
+        pass_filenames: false