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/automations/actions/builtin/agent_task/invoke.py

@@ -11,8 +11,11 @@ from langgraph.types import Command
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.agents.multi_agent_chat import create_multi_agent_chat_deep_agent
+from app.agents.multi_agent_chat.shared.mention_resolver import (
+    resolve_mentions,
+    substitute_in_text,
+)
 from app.agents.shared.context import SurfSenseContextSchema
-from app.agents.shared.mention_resolver import resolve_mentions, substitute_in_text
 from app.db import ChatVisibility, async_session_maker
 from app.schemas.new_chat import MentionedDocumentInfo
 

surfsense_backend/app/automations/services/model_policy.py

@@ -39,7 +39,9 @@ def _is_premium_global(kind: ModelKind, config_id: int) -> bool:
 
     cfg: dict | None = None
     if kind == "llm":
-        from app.agents.shared.llm_config import load_global_llm_config_by_id
+        from app.agents.multi_agent_chat.shared.llm_config import (
+            load_global_llm_config_by_id,
+        )
 
         cfg = load_global_llm_config_by_id(config_id)
     elif kind == "image":