Navi Code TUI: fix Phase 4 review critical/medium issues ...

- PermissionEngine: always-deny returns None via check(); is_always_deny() exposed for explicit rejection; default rules for code_exec/ssh_exec/shell; extract_target() made public.
- ShellRunner: set ShellResult.truncated when output is actually truncated.
- File refs: restrict paths to base_dir/home, block sensitive files/dirs, skip binary, support glob brackets, shared guess_language.
- Permission dialog: deny now renders synthetic tool_call before stopping; shell ! commands require permission with always-allow/deny persistence.
- Tests: add permission tests, fix file_refs/shell tests, add __init__.py to fix pytest name collisions, update websocket integration for AgentSessionOrchestrator.

504 passed, 1 skipped. Ruff clean.

Signed-off-by: Eugene Sukhodolskiy <eugene.sukhodolskiy@gmail.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>