[BUG] Plugin update doesn't advance git submodule checkout, resulting in stale plugin cache

Summary

When a marketplace uses git submodules for plugins, the plugin update process pulls the marketplace repo but never runs git submodule update, leaving the submodule checkout at a stale commit. The plugin cache is then populated from this stale checkout with outdated files.

Related to #17293 (initial clone doesn't init submodules), but this is about the update path — even when the submodule was previously initialized, subsequent updates don't advance it.

Steps to Reproduce

1. Have a marketplace repo that uses a git submodule for a plugin
2. Install the marketplace and plugin — submodule gets cloned at commit A
3. Push a new commit B to the plugin repo with changes
4. Update the marketplace repo's submodule pointer to commit B
5. Trigger a plugin update in Claude Code

Expected Behavior

The submodule checkout should advance from commit A to commit B, and the plugin cache should reflect commit B.

Actual Behavior

• The marketplace repo itself updates (submodule pointer moves to commit B)
• The submodule checkout stays at commit A — git submodule update is never run
• The plugin cache is populated from the stale commit A checkout

Evidence

# Marketplace repo correctly references the new commit
$ git -C ~/.claude/plugins/marketplaces/<marketplace> \
    ls-tree HEAD plugins/<plugin>
160000 commit 7d8814e...  plugins/<plugin>   # pointer is correct

# But the submodule is still checked out at the old commit
$ git -C ~/.claude/plugins/marketplaces/<marketplace>/plugins/<plugin> \
    rev-parse HEAD
43d2f01...   # stale

# git submodule status confirms the mismatch (+ prefix = wrong commit)
$ git -C ~/.claude/plugins/marketplaces/<marketplace> \
    submodule status
+43d2f01... plugins/<plugin> (heads/main-4-g43d2f01)

Environment

• Claude Code version: 2.1.41
• OS: macOS (Darwin 24.6.0)

Suggested Fix

After pulling the marketplace repo, run:

git submodule update --init --recursive

This would fix both the initial clone (#17293) and the update path.

Workaround

Manually update the submodule:

cd ~/.claude/plugins/marketplaces/<marketplace>
git submodule update --init --recursive

Then reinstall or update the plugin so the cache is refreshed.