test(agents): cover live filesystem middleware, retire dead twin

The single-agent-era filesystem middleware (app/agents/shared/middleware/
filesystem.py, ~2000 lines) was never instantiated in production, yet three
unit suites validated it — an illusory guardrail while the live decomposed
middleware (multi_agent_chat/middleware/shared/filesystem) was unguarded.

Close the gap before reorganizing the agents module:
- Add 14 integration tests driving live B's tools in desktop mode (real
  on-disk effects) and cloud mode (in-state staging, namespace policy).
- Port all high-value dead-twin assertions onto the live path: cloud rm/rmdir
  staging + guard rails, KBPostgresBackend delete-view filter, mode-scoped
  system prompt, cwd/relative/namespace resolution, multi-root mount
  normalization.
- Delete dead twin filesystem.py, drop its __init__ re-export, and retire its
  3 dead-twin tests.

Verified: test_import_all + middleware unit + FS integration all green.

surfsense_backend/app/agents/shared/middleware/__init__.py

@@ -21,9 +21,6 @@ from app.agents.shared.middleware.doom_loop import DoomLoopMiddleware
 from app.agents.shared.middleware.file_intent import (
     FileIntentMiddleware,
 )
-from app.agents.shared.middleware.filesystem import (
-    SurfSenseFilesystemMiddleware,
-)
 from app.agents.shared.middleware.flatten_system import (
     FlattenSystemMessageMiddleware,
 )
@@ -78,7 +75,6 @@ __all__ = [
     "SpillToBackendEdit",
     "SpillingContextEditingMiddleware",
     "SurfSenseCompactionMiddleware",
-    "SurfSenseFilesystemMiddleware",
     "ToolCallNameRepairMiddleware",
     "build_skills_backend_factory",
     "commit_staged_filesystem_state",