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/integration/agents/multi_agent_chat/test_kb_filesystem_cloud.py

@@ -20,7 +20,7 @@ from langchain.agents import create_agent
 from langchain_core.messages import HumanMessage, ToolMessage
 from langgraph.checkpoint.memory import InMemorySaver
 
-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/integration/agents/multi_agent_chat/test_kb_filesystem_desktop.py

@@ -1,6 +1,6 @@
 """Real-behavior tests for the LIVE knowledge-base filesystem middleware (B).
 
-These exercise ``app.agents.multi_agent_chat.middleware.shared.filesystem`` —
+These exercise ``app.agents.multi_agent_chat.shared.middleware.filesystem`` —
 the decomposed middleware + tools that production actually mounts on the
 knowledge_base subagent (via ``build_filesystem_mw``). The previous
 ``tests/unit/middleware/test_filesystem_*.py`` suite asserts a *dead twin*
@@ -23,7 +23,7 @@ from langchain.agents import create_agent
 from langchain_core.messages import HumanMessage, ToolMessage
 from langgraph.checkpoint.memory import InMemorySaver
 
-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