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>

Navi Code: Phase 2 — CLI terminal client, tests, docs ...

- Add clients/terminal package: config, state, REST API wrappers, WebSocket
  client, renderer, and click-based interactive CLI.
- Wire navi-code console script via pyproject.toml.
- Add unit and WebSocket integration tests for the terminal client.
- Update docs/profiles.md, docs/config.md, README.md with navi_code profile
  and default-profile instructions.
- Add docs/navi_code.md setup guide and docs/navi_code_cli.md usage reference.
- Fix lint in new test files and test_auth_disabled.py.

Tested: 459 passed, 1 skipped (excluded unrelated websocket test).

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