Navi Code TUI: Phase 5.2 — SessionsPanel, /export, integration tests ...

- Add right-side SessionsPanel widget (DataTable) listing server sessions.
- Wire SessionsPanel into TUI layout and session selection flow.
- Add SessionSelected / SessionListUpdated events and refresh logic.
- Implement /export slash command: markdown rendering + $EDITOR.
- Update /new, /sessions, /switch, /profile to use session_id/name/preview.
- Add integration tests for SessionsPanel and /export; update layout test.
- Update docs/navi_code_cli.md with new panel and command.

Co-Authored-By: Claude <noreply@anthropic.com>