fix(cli): sync footer branch name on filesystems without fs.watch events

[manumishra12]

Summary

The footer Branch indicator does not update after a git checkout on filesystems where fs.watch delivers no change events — most notably WSL mounts of Windows drives (/mnt/c/...) and network shares. The displayed branch stays stuck on whatever was checked out when the CLI started.

Fixes #27957

Details

useGitBranchName already watches the git directory with fs.watch and re-fetches the branch when HEAD changes. But fs.watch relies on the OS notification layer (inotify on Linux, FSEvents on macOS), and on WSL mounts of Windows drives and on many network filesystems that layer delivers no events at all — so the watcher silently never fires and the branch is never refreshed (the environment in #27957 is WSL).

This adds a stat-based fs.watchFile poll on HEAD as a reliable fallback that works regardless of the underlying filesystem, running alongside the existing fs.watch (which remains the fast, event-driven path where it works):

• Both paths funnel into a single debounced scheduleRefresh() helper (no behavior change to the existing fast path).
• The poll only refreshes when HEAD's mtime actually changes, so steady state does no work.
• Poll interval is 2s (a single stat of a tiny file; negligible cost).
• The poll is torn down on unmount via fs.unwatchFile.

No changes are needed in the footer component or UI state — they already react to whatever the hook returns.

How to Validate

Manual (on WSL with the repo under /mnt/<drive>, on a network share, or any affected FS):

1. Start gemini in a git repo and note the branch shown in the footer.
2. Switch branches, e.g. ! git checkout -b some-branch (or in another shell).
3. Expected: within ~2s the footer reflects the new branch. Before this change it stayed on the old branch on affected filesystems.

Automated:

npm run --workspace packages/cli test -- src/ui/hooks/useGitBranchName.test.tsx

A new test covers the polling fallback (when fs.watch never fires), and the cleanup test is extended to assert the poll is removed on unmount.

Pre-Merge Checklist

• Prettier, ESLint (--max-warnings 0) and TypeScript typecheck pass for the changed files.
• Unit tests added and passing (useGitBranchName.test.tsx, plus Footer.test.tsx consumers).
• Change is scoped to a single hook; no public API changes.
• Linked issue: Checkout git branch not reflected in CLI UI #27957

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where the CLI's git branch indicator fails to update after a checkout on specific filesystems that do not support standard OS-level file watch events. By introducing a lightweight stat-based polling fallback for the HEAD file, the branch name now reliably reflects the current state across all environments, including WSL and network-mounted drives, without impacting the performance of existing event-driven platforms.

Highlights

• Reliable Git Branch Tracking: Implemented a fallback polling mechanism using fs.watchFile on the HEAD file to ensure the git branch indicator updates correctly on filesystems where fs.watch events are not triggered, such as WSL mounts and network shares.
• Efficient Polling Strategy: The new polling mechanism only triggers a refresh when the mtime of the HEAD file changes, ensuring minimal performance impact while maintaining a responsive UI.
• Improved Test Coverage: Added comprehensive unit tests to verify the polling fallback behavior and ensure proper cleanup of watchers upon component unmount.

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[github-actions]

📊 PR Size: size/M

• Lines changed: 119
• Additions: +112
• Deletions: -7
• Files changed: 2

[gemini-code-assist]

Code Review

This pull request introduces a polling fallback mechanism using fs.watchFile on the Git HEAD file to ensure the branch name updates correctly on filesystems where fs.watch does not deliver events (such as WSL mounts or network shares). It also adds corresponding unit tests to verify this behavior and ensure proper cleanup on unmount. The review feedback highlights a critical issue where calling fs.unwatchFile with only the file path removes all listeners for that file across all active hook instances. To prevent this, the reviewer suggests storing and passing the specific listener function to fs.unwatchFile during cleanup, and updating the tests to assert this behavior.