feat(coding-agent/cli): added foldable git TUI sidebar sections and persistent error status

- Implemented collapsible section headers that toggle folding on click, Enter, or arrow keys, and updated UI to reflect folded state.
- Added persistent single-line error status handling via #setError, preventing transient error messages from disappearing.
- Updated Sidebar to manage collapsed sections, introduced SectionTarget type, and added tests verifying folding behavior and header pill actions.