refactor(agents): move mac-only modules out of the cross-agent shared kernel

app/agents/shared/ is a sibling of anonymous_chat/podcaster/multi_agent_chat/
video_presentation, so it should only hold code shared across 2+ of those
agents. In practice podcaster and video_presentation import nothing from it,
and anonymous_chat needs only context + compaction + retry_after + web_search.
Everything else was multi_agent_chat-only (the boundary just passes through).

Move the multi_agent_chat-only cluster into multi_agent_chat/shared/ (files
moved verbatim via git rename; ~116 import sites rewritten):

  errors, feature_flags, filesystem_selection, path_resolver, prompt_caching,
  sandbox, llm_config, mention_resolver
  middleware/busy_mutex, middleware/kb_persistence

busy_mutex/llm_config/mention_resolver are boundary-only but import the moved
modules, so they were folded in to avoid a backwards shared -> multi_agent_chat
dependency. main_agent builders now import the impls directly; the shared
middleware barrel keeps only the genuinely-shared compaction + retry_after.

Also delete the dead leftover shared/plugins and shared/skills dirs (live
copies already live under main_agent/).

Remaining in app/agents/shared/: context, system_prompt(+prompts), checkpointer,
middleware/{compaction,retry_after,dedup_tool_calls}, tools/. checkpointer and
system_prompt are boundary-only infra pending a dedicated home decision.

surfsense_backend/app/routes/agent_action_log_route.py

@@ -28,7 +28,7 @@ from pydantic import BaseModel
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.shared.feature_flags import get_flags
+from app.agents.multi_agent_chat.shared.feature_flags import get_flags
 from app.db import (
     AgentActionLog,
     NewChatThread,

surfsense_backend/app/routes/agent_flags_route.py

@@ -22,7 +22,10 @@ from dataclasses import asdict
 from fastapi import APIRouter, Depends
 from pydantic import BaseModel
 
-from app.agents.shared.feature_flags import AgentFeatureFlags, get_flags
+from app.agents.multi_agent_chat.shared.feature_flags import (
+    AgentFeatureFlags,
+    get_flags,
+)
 from app.config import config
 from app.db import User
 from app.users import current_active_user

surfsense_backend/app/routes/agent_permissions_route.py

@@ -30,7 +30,7 @@ from sqlalchemy import select
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.shared.feature_flags import get_flags
+from app.agents.multi_agent_chat.shared.feature_flags import get_flags
 from app.db import (
     AgentPermissionRule,
     NewChatThread,

surfsense_backend/app/routes/agent_revert_route.py

@@ -32,7 +32,7 @@ from sqlalchemy import select
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.shared.feature_flags import get_flags
+from app.agents.multi_agent_chat.shared.feature_flags import get_flags
 from app.db import (
     AgentActionLog,
     User,

surfsense_backend/app/routes/anonymous_chat_routes.py

@@ -236,7 +236,7 @@ async def stream_anonymous_chat(
             detail="No-login mode is not enabled.",
         )
 
-    from app.agents.shared.llm_config import (
+    from app.agents.multi_agent_chat.shared.llm_config import (
         AgentConfig,
         create_chat_litellm_from_agent_config,
     )

surfsense_backend/app/routes/documents_routes.py

@@ -7,7 +7,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.future import select
 from sqlalchemy.orm import selectinload
 
-from app.agents.shared.path_resolver import virtual_path_to_doc
+from app.agents.multi_agent_chat.shared.path_resolver import virtual_path_to_doc
 from app.db import (
     Chunk,
     Document,

surfsense_backend/app/routes/new_chat_routes.py

@@ -24,13 +24,13 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.future import select
 from sqlalchemy.orm import selectinload
 
-from app.agents.shared.filesystem_selection import (
+from app.agents.multi_agent_chat.shared.filesystem_selection import (
     ClientPlatform,
     FilesystemMode,
     FilesystemSelection,
     LocalFilesystemMount,
 )
-from app.agents.shared.middleware.busy_mutex import (
+from app.agents.multi_agent_chat.shared.middleware.busy_mutex import (
     get_cancel_state,
     is_cancel_requested,
     manager,
@@ -476,7 +476,7 @@ async def _revert_turns_for_regenerate(
 
 def _try_delete_sandbox(thread_id: int) -> None:
     """Fire-and-forget sandbox + local file deletion so the HTTP response isn't blocked."""
-    from app.agents.shared.sandbox import (
+    from app.agents.multi_agent_chat.shared.sandbox import (
         delete_local_sandbox_files,
         delete_sandbox,
         is_sandbox_enabled,

surfsense_backend/app/routes/sandbox_routes.py

@@ -51,7 +51,10 @@ async def download_sandbox_file(
 ):
     """Download a file from the Daytona sandbox associated with a chat thread."""
 
-    from app.agents.shared.sandbox import get_or_create_sandbox, is_sandbox_enabled
+    from app.agents.multi_agent_chat.shared.sandbox import (
+        get_or_create_sandbox,
+        is_sandbox_enabled,
+    )
 
     if not is_sandbox_enabled():
         raise HTTPException(status_code=404, detail="Sandbox is not enabled")
@@ -71,7 +74,7 @@ async def download_sandbox_file(
         "You don't have permission to access files in this thread",
     )
 
-    from app.agents.shared.sandbox import get_local_sandbox_file
+    from app.agents.multi_agent_chat.shared.sandbox import get_local_sandbox_file
 
     # Prefer locally-persisted copy (sandbox may already be deleted)
     local_content = get_local_sandbox_file(thread_id, path)