Unify the changes list, pull request details, and commit history into
a single multi-segment panel. This improves navigation and state sharing
across different lenses of a worktree's source control.
- Add a segmented control to switch between Changes, PR, and History
- Introduce a persistent branch status card with an integrated PR chip
- Redirect standalone PR and history routes to the new unified hub
- Extract reusable UI and logic for the history list and PR summary
