fix: path traversal guard, rollback extension re-registration, lifecycle docs

- remove_tracked_files: validate resolved path stays within project_path
  before unlinking; reject entries with '../' that escape the project root
- Rollback: call _reregister_extension_commands() during rollback (same
  as success path) so extension files are properly restored
- AgentBootstrap: comprehensive lifecycle flow docstring documenting the
  setup → finalize_setup → get_tracked_files → check_modified → teardown
  chain and explaining why tracking all files is safe (hash check)

src/specify_cli/__init__.py

@@ -2740,14 +2740,12 @@ def agent_switch(
                 rollback_resolved = resolve_agent_pack(current_agent, project_path=project_path)
                 rollback_bs = load_bootstrap(rollback_resolved.path, rollback_resolved.manifest)
                 rollback_files = rollback_bs.setup(project_path, script_type, options)
+                # Re-register extension commands (same as the success path)
+                rollback_ext_files = _reregister_extension_commands(project_path, current_agent)
                 rollback_bs.finalize_setup(
                     project_path,
                     agent_files=rollback_files,
-                    extension_files=list(
-                        (project_path / p).resolve()
-                        for p in old_tracked_ext
-                        if (project_path / p).is_file()
-                    ),
+                    extension_files=rollback_ext_files,
                 )
                 console.print(f"  [green]✓[/green] {current_agent} restored")
             except Exception: