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/tasks/chat/streaming/agent/builder.py

@@ -9,8 +9,8 @@ from __future__ import annotations
 
 from typing import Any
 
-from app.agents.shared.filesystem_selection import FilesystemSelection
-from app.agents.shared.llm_config import AgentConfig
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemSelection
+from app.agents.multi_agent_chat.shared.llm_config import AgentConfig
 from app.db import ChatVisibility
 from app.services.connector_service import ConnectorService
 

surfsense_backend/app/tasks/chat/streaming/agent/event_loop.py

@@ -11,8 +11,8 @@ from __future__ import annotations
 from collections.abc import AsyncGenerator
 from typing import Any
 
-from app.agents.shared.filesystem_selection import FilesystemMode
-from app.agents.shared.middleware.kb_persistence import (
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemMode
+from app.agents.multi_agent_chat.shared.middleware.kb_persistence import (
     commit_staged_filesystem_state,
 )
 from app.services.new_streaming_service import VercelStreamingService

surfsense_backend/app/tasks/chat/streaming/errors/classifier.py

@@ -7,8 +7,8 @@ import logging
 import time
 from typing import Any, Literal
 
-from app.agents.shared.errors import BusyError
-from app.agents.shared.middleware.busy_mutex import (
+from app.agents.multi_agent_chat.shared.errors import BusyError
+from app.agents.multi_agent_chat.shared.middleware.busy_mutex import (
     get_cancel_state,
     is_cancel_requested,
 )

surfsense_backend/app/tasks/chat/streaming/flows/new_chat/input_state.py

@@ -28,8 +28,11 @@ from langchain_core.messages import HumanMessage
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.future import select
 
-from app.agents.shared.filesystem_selection import FilesystemMode
-from app.agents.shared.mention_resolver import resolve_mentions, substitute_in_text
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemMode
+from app.agents.multi_agent_chat.shared.mention_resolver import (
+    resolve_mentions,
+    substitute_in_text,
+)
 from app.db import (
     ChatVisibility,
     NewChatThread,

surfsense_backend/app/tasks/chat/streaming/flows/new_chat/llm_capability.py

@@ -15,7 +15,7 @@ tells the user what to change.
 
 from __future__ import annotations
 
-from app.agents.shared.llm_config import AgentConfig
+from app.agents.multi_agent_chat.shared.llm_config import AgentConfig
 from app.observability import otel as ot
 
 

surfsense_backend/app/tasks/chat/streaming/flows/new_chat/orchestrator.py

@@ -30,8 +30,11 @@ from typing import Any, Literal
 import anyio
 
 from app.agents.multi_agent_chat import create_multi_agent_chat_deep_agent
-from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
-from app.agents.shared.middleware.busy_mutex import end_turn
+from app.agents.multi_agent_chat.shared.filesystem_selection import (
+    FilesystemMode,
+    FilesystemSelection,
+)
+from app.agents.multi_agent_chat.shared.middleware.busy_mutex import end_turn
 from app.db import ChatVisibility, async_session_maker
 from app.observability import otel as ot
 from app.services.new_streaming_service import VercelStreamingService
@@ -826,7 +829,7 @@ async def stream_new_chat(
         # downloadable after the Daytona sandbox auto-deletes.
         if stream_result and stream_result.sandbox_files:
             with contextlib.suppress(Exception):
-                from app.agents.shared.sandbox import (
+                from app.agents.multi_agent_chat.shared.sandbox import (
                     is_sandbox_enabled,
                     persist_and_delete_sandbox,
                 )

surfsense_backend/app/tasks/chat/streaming/flows/new_chat/title_gen.py

@@ -30,7 +30,7 @@ from app.prompts import TITLE_GENERATION_PROMPT
 from app.services.new_streaming_service import VercelStreamingService
 
 if TYPE_CHECKING:
-    from app.agents.shared.llm_config import AgentConfig
+    from app.agents.multi_agent_chat.shared.llm_config import AgentConfig
     from app.services.token_tracking_service import TokenAccumulator
 
 

surfsense_backend/app/tasks/chat/streaming/flows/resume_chat/orchestrator.py

@@ -24,8 +24,11 @@ from uuid import UUID
 import anyio
 
 from app.agents.multi_agent_chat import create_multi_agent_chat_deep_agent
-from app.agents.shared.filesystem_selection import FilesystemMode, FilesystemSelection
-from app.agents.shared.middleware.busy_mutex import end_turn
+from app.agents.multi_agent_chat.shared.filesystem_selection import (
+    FilesystemMode,
+    FilesystemSelection,
+)
+from app.agents.multi_agent_chat.shared.middleware.busy_mutex import end_turn
 from app.db import ChatVisibility, async_session_maker
 from app.observability import otel as ot
 from app.services.chat_session_state_service import set_ai_responding

surfsense_backend/app/tasks/chat/streaming/flows/shared/llm_bundle.py

@@ -14,7 +14,7 @@ from typing import Any
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.shared.llm_config import (
+from app.agents.multi_agent_chat.shared.llm_config import (
     AgentConfig,
     create_chat_litellm_from_agent_config,
     create_chat_litellm_from_config,

surfsense_backend/app/tasks/chat/streaming/flows/shared/premium_quota.py

@@ -19,7 +19,7 @@ from dataclasses import dataclass
 from typing import TYPE_CHECKING
 from uuid import UUID
 
-from app.agents.shared.llm_config import AgentConfig
+from app.agents.multi_agent_chat.shared.llm_config import AgentConfig
 from app.db import shielded_async_session
 
 if TYPE_CHECKING:

surfsense_backend/app/tasks/chat/streaming/flows/shared/rate_limit_recovery.py

@@ -17,7 +17,7 @@ from typing import Literal
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.shared.middleware.busy_mutex import end_turn
+from app.agents.multi_agent_chat.shared.middleware.busy_mutex import end_turn
 from app.observability import otel as ot
 from app.services.auto_model_pin_service import (
     mark_runtime_cooldown,

surfsense_backend/app/tasks/chat/streaming/flows/shared/stream_loop.py

@@ -15,7 +15,7 @@ from __future__ import annotations
 from collections.abc import AsyncGenerator, Awaitable, Callable
 from typing import Any
 
-from app.agents.shared.filesystem_selection import FilesystemMode
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemMode
 from app.services.new_streaming_service import VercelStreamingService
 from app.tasks.chat.streaming.agent.event_loop import stream_agent_events
 from app.tasks.chat.streaming.shared.stream_result import StreamResult

surfsense_backend/app/tasks/chat/streaming/flows/shared/terminal_error.py

@@ -14,7 +14,7 @@ import traceback
 from collections.abc import Iterator
 from typing import Any, Literal
 
-from app.agents.shared.errors import BusyError
+from app.agents.multi_agent_chat.shared.errors import BusyError
 from app.observability import metrics as ot_metrics, otel as ot
 from app.services.new_streaming_service import VercelStreamingService
 from app.tasks.chat.streaming.errors.classifier import classify_stream_exception