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/agents/multi_agent_chat/subagents/builtins/knowledge_base/agent.py

@@ -13,9 +13,9 @@ from deepagents import SubAgent
 from langchain_core.language_models import BaseChatModel
 from langchain_core.tools import BaseTool
 
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemMode
 from app.agents.multi_agent_chat.shared.permissions import Rule, Ruleset
 from app.agents.multi_agent_chat.subagents.shared.spec import SurfSenseSubagentSpec
-from app.agents.shared.filesystem_selection import FilesystemMode
 
 from .middleware_stack import build_kb_middleware
 from .prompts import load_description, load_readonly_system_prompt, load_system_prompt

surfsense_backend/app/agents/multi_agent_chat/subagents/builtins/knowledge_base/middleware_stack.py

@@ -10,6 +10,8 @@ from typing import Any
 
 from langchain_core.language_models import BaseChatModel
 
+from app.agents.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemMode
 from app.agents.multi_agent_chat.shared.middleware.anthropic_cache import (
     build_anthropic_cache_mw,
 )
@@ -29,8 +31,6 @@ from app.agents.multi_agent_chat.shared.middleware.permissions import (
     build_permission_mw,
 )
 from app.agents.multi_agent_chat.shared.permissions import Ruleset
-from app.agents.shared.feature_flags import AgentFeatureFlags
-from app.agents.shared.filesystem_selection import FilesystemMode
 
 
 def _kb_user_allowlist(

surfsense_backend/app/agents/multi_agent_chat/subagents/builtins/knowledge_base/prompts.py

@@ -2,8 +2,8 @@
 
 from __future__ import annotations
 
+from app.agents.multi_agent_chat.shared.filesystem_selection import FilesystemMode
 from app.agents.multi_agent_chat.subagents.shared.md_file_reader import read_md_file
-from app.agents.shared.filesystem_selection import FilesystemMode
 
 
 def load_system_prompt(filesystem_mode: FilesystemMode) -> str:

surfsense_backend/app/agents/multi_agent_chat/subagents/shared/middleware/middleware_stack.py

@@ -14,8 +14,7 @@ from __future__ import annotations
 
 from typing import Any
 
-from app.agents.shared.feature_flags import AgentFeatureFlags
-
+from app.agents.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
 from app.agents.multi_agent_chat.shared.middleware.permissions import (
     build_permission_mw,
 )