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/tests/unit/tasks/chat/streaming/test_interrupt_inspector_all.py

@@ -18,7 +18,7 @@ from langgraph.graph import END, START, StateGraph
 from langgraph.types import Send, interrupt
 from typing_extensions import TypedDict
 
-from app.agents.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
+from app.agents.chat.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
 from app.tasks.chat.streaming.helpers.interrupt_inspector import (