feat: live-update git branch in TUI footers

[rumpl]

Summary

• add a shared event-driven Git branch watcher using fsnotify
• update the lean TUI footer when HEAD changes
• reuse the watcher in the full TUI sidebar
• support repositories and linked worktrees without polling

Testing

• task build
• task lint
• go test ./pkg/gitbranch ./pkg/leantui/... ./pkg/tui/components/sidebar

Notes

• The full task test run has an environment-dependent failure in pkg/sandbox (TestExtraWorkspace/built-in_name resolves the main worktree examples path). All other packages passed.