fix(app): refresh review panel on external git commits

[lxqddd]

The review panel caches its vcs.diff query with every refetch hook disabled, so it only refreshes through explicit invalidation. Most terminal git operations still refresh it by accident because they rewrite workspace files, but a git commit/add/amend only rewrites files under .git, which produced no event:

• the server's .git subscription watches only HEAD, and a regular commit does not change HEAD's content
• the client's session VCS listener dropped every .git path anyway (that check also compared a relative prefix against absolute event paths, so it never actually matched)

The panel then kept showing pre-commit changes until an unrelated event happened to refresh it.

• watch .git/index alongside HEAD in the core watcher (staging and commits rewrite it; everything else under .git stays ignored)
• in the session VCS listener, treat paths under .git whose basename is index or HEAD as VCS state changes and call the existing debounced refreshVcs(); basename matching also covers git worktrees, where the path is .git/worktrees//index
• flip the watcher test to assert .git/index events are published and add coverage that other .git files remain ignored

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