refactor(agents): consolidate chat runtime infra under chat/runtime

Move the lower-level runtime/infra modules out of multi_agent_chat/shared/
(they were never used by subagents, so they failed the shared-by-all-siblings
rule) and unify them with the already-relocated checkpointer:

  agents/runtime/                      -> agents/chat/runtime/
  mac/shared/errors.py                 -> chat/runtime/errors.py
  mac/shared/llm_config.py             -> chat/runtime/llm_config.py
  mac/shared/prompt_caching.py         -> chat/runtime/prompt_caching.py
  mac/shared/mention_resolver.py       -> chat/runtime/mention_resolver.py
  mac/shared/path_resolver.py          -> chat/runtime/path_resolver.py

These sit below the agent packages: the boundary + agent factory + shared
middleware depend on them, and they import no agent code (acyclic).

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

@@ -12,7 +12,7 @@ from typing import Any
 from app.agents.chat.multi_agent_chat.shared.filesystem_selection import (
     FilesystemSelection,
 )
-from app.agents.chat.multi_agent_chat.shared.llm_config import AgentConfig
+from app.agents.chat.runtime.llm_config import AgentConfig
 from app.db import ChatVisibility
 from app.services.connector_service import ConnectorService
 

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

@@ -7,11 +7,11 @@ import logging
 import time
 from typing import Any, Literal
 
-from app.agents.chat.multi_agent_chat.shared.errors import BusyError
 from app.agents.chat.multi_agent_chat.shared.middleware.busy_mutex import (
     get_cancel_state,
     is_cancel_requested,
 )
+from app.agents.chat.runtime.errors import BusyError
 
 TURN_CANCELLING_INITIAL_DELAY_MS = 200
 TURN_CANCELLING_BACKOFF_FACTOR = 2

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

@@ -29,7 +29,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.future import select
 
 from app.agents.chat.multi_agent_chat.shared.filesystem_selection import FilesystemMode
-from app.agents.chat.multi_agent_chat.shared.mention_resolver import (
+from app.agents.chat.runtime.mention_resolver import (
     resolve_mentions,
     substitute_in_text,
 )

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.chat.multi_agent_chat.shared.llm_config import AgentConfig
+from app.agents.chat.runtime.llm_config import AgentConfig
 from app.observability import otel as ot
 
 

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.chat.multi_agent_chat.shared.llm_config import AgentConfig
+    from app.agents.chat.runtime.llm_config import AgentConfig
     from app.services.token_tracking_service import TokenAccumulator
 
 

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.chat.multi_agent_chat.shared.llm_config import (
+from app.agents.chat.runtime.llm_config import (
     AgentConfig,
     create_chat_litellm_from_agent_config,
     create_chat_litellm_from_config,

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

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.agents.runtime.checkpointer import get_checkpointer
+from app.agents.chat.runtime.checkpointer import get_checkpointer
 from app.db import SearchSourceConnectorType
 from app.services.connector_service import ConnectorService
 
@@ -33,7 +33,7 @@ async def setup_connector_and_firecrawl(
 async def get_chat_checkpointer():
     """Resolve the PostgreSQL checkpointer for persistent conversation memory.
 
-    Thin wrapper around ``app.agents.runtime.checkpointer.get_checkpointer`` so
+    Thin wrapper around ``app.agents.chat.runtime.checkpointer.get_checkpointer`` so
     flow orchestrators can rely on a streaming-local symbol and we have a hook
     point if the checkpointer source ever needs to vary per flow.
     """

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.chat.multi_agent_chat.shared.llm_config import AgentConfig
+from app.agents.chat.runtime.llm_config import AgentConfig
 from app.db import shielded_async_session
 
 if TYPE_CHECKING:

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.chat.multi_agent_chat.shared.errors import BusyError
+from app.agents.chat.runtime.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