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/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_permission_ask_mcp_context.py

@@ -13,6 +13,7 @@ from langgraph.graph.message import add_messages
 from pydantic import BaseModel
 from typing_extensions import TypedDict
 
+from app.agents.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
 from app.agents.multi_agent_chat.shared.middleware.permissions import (
     build_permission_mw,
 )
@@ -20,7 +21,6 @@ from app.agents.multi_agent_chat.shared.middleware.permissions.ask.payload impor
     build_permission_ask_payload,
 )
 from app.agents.multi_agent_chat.shared.permissions import Rule, Ruleset
-from app.agents.shared.feature_flags import AgentFeatureFlags
 
 
 class _NoArgs(BaseModel):

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_subagent_owned_ruleset.py

@@ -23,11 +23,11 @@ from langgraph.graph.message import add_messages
 from langgraph.types import Command
 from typing_extensions import TypedDict
 
+from app.agents.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
 from app.agents.multi_agent_chat.shared.middleware.permissions import (
     build_permission_mw,
 )
 from app.agents.multi_agent_chat.shared.permissions import Rule, Ruleset
-from app.agents.shared.feature_flags import AgentFeatureFlags
 
 
 def _kb_style_ruleset() -> Ruleset:

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_trusted_tool_save_on_always.py

@@ -14,11 +14,11 @@ from langgraph.types import Command
 from pydantic import BaseModel
 from typing_extensions import TypedDict
 
+from app.agents.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
 from app.agents.multi_agent_chat.shared.middleware.permissions import (
     build_permission_mw,
 )
 from app.agents.multi_agent_chat.shared.permissions import Rule, Ruleset
-from app.agents.shared.feature_flags import AgentFeatureFlags
 
 
 class _NoArgs(BaseModel):

surfsense_backend/tests/unit/agents/multi_agent_chat/subagents/shared/test_subagent_builder.py

@@ -19,6 +19,7 @@ from langchain_core.language_models.fake_chat_models import (
 from langchain_core.messages import AIMessage, BaseMessage, HumanMessage
 from langchain_core.outputs import ChatGeneration, ChatResult
 
+from app.agents.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
 from app.agents.multi_agent_chat.shared.middleware.permissions.middleware.core import (
     PermissionMiddleware,
 )
@@ -26,7 +27,6 @@ from app.agents.multi_agent_chat.shared.permissions import Rule, Ruleset, evalua
 from app.agents.multi_agent_chat.subagents.shared.subagent_builder import (
     pack_subagent,
 )
-from app.agents.shared.feature_flags import AgentFeatureFlags
 
 
 class RateLimitError(Exception):