refactor(agents): move filesystem_state, path_resolver, sandbox to app/agents/shared (slice 3b)

Relocate three leaf filesystem-cluster modules to the shared kernel and flip
all 38 importers. No re-export shims needed (no frozen single-agent importer).
This also resolves the pre-existing shared->new_chat back-edge from
shared/receipt_command.py onto filesystem_state.

filesystem_backends is intentionally deferred to slice 5: it depends on
new_chat middleware (kb_postgres_backend, multi_root_local_folder_backend)
that have not yet moved, so relocating it now would create a shared->new_chat edge.

surfsense_backend/tests/unit/agents/new_chat/test_mention_resolver.py

@@ -22,7 +22,7 @@ from app.agents.new_chat.mention_resolver import (
     resolve_mentions,
     substitute_in_text,
 )
-from app.agents.new_chat.path_resolver import DOCUMENTS_ROOT, PathIndex
+from app.agents.shared.path_resolver import DOCUMENTS_ROOT, PathIndex
 from app.schemas.new_chat import MentionedDocumentInfo
 
 pytestmark = pytest.mark.unit

surfsense_backend/tests/unit/agents/new_chat/test_path_resolver.py

@@ -7,7 +7,7 @@ from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 
-from app.agents.new_chat.path_resolver import (
+from app.agents.shared.path_resolver import (
     DOCUMENTS_ROOT,
     PathIndex,
     doc_to_virtual_path,

surfsense_backend/tests/unit/middleware/test_knowledge_tree.py

@@ -10,7 +10,7 @@ contract cannot silently regress.
 from __future__ import annotations
 
 from app.agents.new_chat.middleware.knowledge_tree import KnowledgeTreeMiddleware
-from app.agents.new_chat.path_resolver import DOCUMENTS_ROOT
+from app.agents.shared.path_resolver import DOCUMENTS_ROOT
 
 
 def _compute(folder_paths: list[str], doc_paths: list[str]) -> set[str]:
@@ -86,7 +86,7 @@ class TestFormatTreeRendering:
         folder_paths: list[str],
         doc_specs: list[dict],
     ) -> str:
-        from app.agents.new_chat.path_resolver import PathIndex
+        from app.agents.shared.path_resolver import PathIndex
 
         index = PathIndex(
             folder_paths={i + 1: p for i, p in enumerate(folder_paths)},