refactor(agents): introduce chat/ category; dissolve top-level agents/shared

Recursive shared-folder rule: a shared/ must be shared by ALL siblings at its
level. The kernel (context, compaction, retry_after, web_search) was shared by
only 2 of the agents -- anonymous_chat + multi_agent_chat -- never by podcaster
or video_presentation. Those 2 are the "chat" category, so their shared code
belongs in that category's shared/, not the top-level one.

  app/agents/anonymous_chat/   -> app/agents/chat/anonymous_chat/
  app/agents/multi_agent_chat/ -> app/agents/chat/multi_agent_chat/
  app/agents/shared/           -> app/agents/chat/shared/   (anon<->mac kernel)

Top-level app/agents/shared/ is gone: nothing was shared across all three
categories (chat / podcaster / video_presentation).

~289 import sites rewritten (app.agents.{anonymous_chat,multi_agent_chat,shared}
-> app.agents.chat.*); all moves are git renames (history preserved).
app/agents/ now: chat/, podcaster/, video_presentation/, runtime/.

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.multi_agent_chat.shared.llm_config import (
+from app.agents.chat.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.multi_agent_chat.shared.llm_config import AgentConfig
+from app.agents.chat.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.multi_agent_chat.shared.middleware.busy_mutex import end_turn
+from app.agents.chat.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.multi_agent_chat.shared.filesystem_selection import FilesystemMode
+from app.agents.chat.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.multi_agent_chat.shared.errors import BusyError
+from app.agents.chat.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