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/middleware/test_knowledge_tree.py

@@ -9,10 +9,10 @@ contract cannot silently regress.
 
 from __future__ import annotations
 
-from app.agents.multi_agent_chat.main_agent.middleware.knowledge_tree.middleware import (
+from app.agents.chat.multi_agent_chat.main_agent.middleware.knowledge_tree.middleware import (
     KnowledgeTreeMiddleware,
 )
-from app.agents.multi_agent_chat.shared.path_resolver import DOCUMENTS_ROOT
+from app.agents.chat.multi_agent_chat.shared.path_resolver import DOCUMENTS_ROOT
 
 
 def _compute(folder_paths: list[str], doc_paths: list[str]) -> set[str]:
@@ -88,7 +88,7 @@ class TestFormatTreeRendering:
         folder_paths: list[str],
         doc_specs: list[dict],
     ) -> str:
-        from app.agents.multi_agent_chat.shared.path_resolver import PathIndex
+        from app.agents.chat.multi_agent_chat.shared.path_resolver import PathIndex
 
         index = PathIndex(
             folder_paths={i + 1: p for i, p in enumerate(folder_paths)},