Move terminal_manager to _internal subpackage ...

- terminal_manager is an internal helper, not a tool
- Update imports in terminal.py, container.py, test_terminal.py

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

Fix terminal review issues: security, lifecycle, reactivity ...

- TerminalManager.open now accepts exec_tokens to use create_subprocess_exec
  for restricted commands instead of always using shell
- Fix kill/terminate order: SIGTERM first, SIGKILL fallback
- Pop closed sessions from _sessions dict to prevent memory leak
- Add terminal_manager.shutdown() to AppContainer.shutdown()
- Wait for reader tasks in foreground open before returning output
- Add _MAX_TERMINALS_PER_SESSION limit (10)
- Wrap cleanup_idle tasks in _close_one_safe with error logging
- send_input catches BrokenPipeError/ConnectionResetError specifically
- Foreground terminals auto-close after gathering output
- Vue reactivity: replace terminals object immutably instead of mutating
- onTerminalClosed marks matching tool card as no longer pending
- Update tests for new behavior (foreground auto-close, max limit)

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

Add persistent multi-session terminal tool with background support ...

- New TerminalManager module: named subprocess sessions per Navi session,
  background readers, event-sink streaming, idle auto-cleanup
- Refactor terminal tool to multi-action: run, open, close, list,
  status, send_input
- Add TerminalOutputDelta and TerminalClosed events for streaming
- Wire TerminalManager into AppContainer, orchestrator, and registry
- Persist session_metadata in Session model and pg_session_store
- Close all session terminals on session delete
- Webclient: handle terminal_output/terminal_closed WS events,
  display live terminal output in tool cards
- Update unit tests for new terminal actions

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

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>