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/integration/google_unification/conftest.py

@@ -239,7 +239,7 @@ def patched_shielded_session(async_engine, monkeypatch):
             yield session
 
     monkeypatch.setattr(
-        "app.agents.multi_agent_chat.subagents.builtins.deliverables.tools.knowledge_base.shielded_async_session",
+        "app.agents.chat.multi_agent_chat.subagents.builtins.deliverables.tools.knowledge_base.shielded_async_session",
         _test_shielded,
     )