Fix WebSocket 403 — bypass FastAPI Depends for WS auth ...

- websocket.py: resolve user by calling get_current_user_ws directly
  inside the handler instead of using Depends(). This avoids FastAPI
  returning HTTP 403 on the upgrade request when auth resolution fails.
- websocket.py: accept WebSocket before access check, close with 4003
  for auth failures instead of HTTP 403.
- auth/deps.py: remove debug logging from get_current_user_ws.
- tests/conftest.py: monkeypatch get_current_user_ws directly since
  Depends() is no longer used on the WebSocket endpoint.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Multi-user auth via gnexus-auth OAuth + hybrid role/permission model ...

- Integrate gnexus-auth-client-py (GAuthClient) for OAuth flow, token refresh,
  and webhook parsing
- Add navi/auth/ package: User model, Fernet encryptor, client singleton,
  deps (get_current_user, require_admin, require_permission)
- New tables: navi_users, user_auth_sessions (auto-created on startup)
- Session/memory isolation by user_id with legacy NULL support
- Cookie-based auth proxy: /auth/login, /callback, /logout, /me
- Webhook receiver /webhooks/gnexus-auth handling user events, global logout,
  session revocation, role/permission changes
- Admin endpoints (/admin/*) gated by role + permissions
- Webclient auth store with isAdmin/hasPermission guards
- Admin-only profile filtering in /agents/profiles
- 200/200 tests passing

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Paginate session list loading

Add 62 tests across all planned phases + fix integration flakiness ...

- Phase 3: 19 API route integration tests (health, agents, sessions, messages)
- Phase 3: 7 WebSocket integration tests (connect, send, replay, invalid, stop)
- Phase 4: 9 agent tests (_check_context_size, _iter_stream_guarded)
- Phase 4: 5 planning tests (_parse_plan_steps)
- Phase 5: 22 tool tests (filesystem 13, code_exec 5, terminal 4)
- Fix flaky integration tests by patching module-level singletons
  (_session_store, _registries, _workers) instead of getter functions,
  because FastAPI Depends() captures the original function at import time.
- Update docs/testing.md coverage table (150 total tests)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>