refactor(agents): colocate middleware into vertical slices

Eliminate the top-level multi_agent_chat/middleware/ package so each slice
owns its middleware (vertical-slice colocation):

- middleware/shared/   -> shared/middleware/        (cross-slice middleware)
- middleware/subagent/ -> subagents/shared/middleware/ (subagent stack)
- main_agent/middleware/ already colocated in Slice A

The moved shared/ subtree is internally consistent (all relative imports
stay within it), so only external absolute refs were rewritten. The
subagent stack's ..shared.* relatives were promoted to absolute paths to
the new shared/middleware/ location.

multi_agent_chat/ root is now: main_agent/, shared/, subagents/.
Verified: 2430 unit tests pass, 1 skipped (baseline unchanged).

surfsense_backend/tests/unit/middleware/test_b_filesystem_path_resolution.py

@@ -8,7 +8,7 @@ Ported from the dead-twin suites:
 
 Both exercised ``app.agents.shared.middleware.filesystem`` (dead). This drives
 the production free functions in
-``app.agents.multi_agent_chat.middleware.shared.filesystem.middleware`` instead.
+``app.agents.multi_agent_chat.shared.middleware.filesystem.middleware`` instead.
 The functions only touch ``mw._filesystem_mode`` and ``mw._get_backend`` so we
 pass a lightweight fake ``mw`` rather than constructing the full middleware.
 """
@@ -20,13 +20,13 @@ from types import SimpleNamespace
 
 import pytest
 
-from app.agents.multi_agent_chat.middleware.shared.filesystem.middleware.mode import (
+from app.agents.multi_agent_chat.shared.middleware.filesystem.middleware.mode import (
     default_cwd,
 )
-from app.agents.multi_agent_chat.middleware.shared.filesystem.middleware.namespace_policy import (
+from app.agents.multi_agent_chat.shared.middleware.filesystem.middleware.namespace_policy import (
     check_cloud_write_namespace,
 )
-from app.agents.multi_agent_chat.middleware.shared.filesystem.middleware.path_resolution import (
+from app.agents.multi_agent_chat.shared.middleware.filesystem.middleware.path_resolution import (
     current_cwd,
     get_contract_suggested_path,
     normalize_local_mount_path,

surfsense_backend/tests/unit/middleware/test_b_filesystem_rm_rmdir_cloud.py

@@ -3,7 +3,7 @@
 Ported from the former ``tests/unit/agents/new_chat/test_rm_rmdir_cloud.py``,
 which exercised the *dead twin* ``app.agents.shared.middleware.filesystem``.
 This drives the production decomposed tools
-(``app.agents.multi_agent_chat.middleware.shared.filesystem``) instead: it
+(``app.agents.multi_agent_chat.shared.middleware.filesystem``) instead: it
 builds the real middleware via ``build_filesystem_mw``, pulls the real ``rm`` /
 ``rmdir`` tools off it, and invokes their coroutines with a stubbed
 ``KBPostgresBackend`` + runtime so we can assert the end-of-turn staging
@@ -19,7 +19,7 @@ from unittest.mock import AsyncMock
 
 import pytest
 
-from app.agents.multi_agent_chat.middleware.shared.filesystem import (
+from app.agents.multi_agent_chat.shared.middleware.filesystem import (
     build_filesystem_mw,
 )
 from app.agents.shared.filesystem_backends import build_backend_resolver

surfsense_backend/tests/unit/middleware/test_b_filesystem_system_prompt.py

@@ -18,7 +18,7 @@ from __future__ import annotations
 
 import pytest
 
-from app.agents.multi_agent_chat.middleware.shared.filesystem.system_prompt import (
+from app.agents.multi_agent_chat.shared.middleware.filesystem.system_prompt import (
     build_system_prompt,
 )
 from app.agents.shared.filesystem_selection import FilesystemMode