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/agents/shared/context.py

@@ -1,71 +0,0 @@
-"""
-Context schema definitions for SurfSense agents.
-
-This module defines the per-invocation context object passed to the SurfSense
-deep agent via ``agent.astream_events(..., context=ctx)`` (LangGraph >= 0.6).
-
-The agent's compiled graph is the same across invocations (and cached by
-``agent_cache``), so anything that varies per turn — the user mentions a
-specific document, the front-end issues a unique ``request_id``, etc. —
-MUST live on this context object instead of being captured into a
-middleware ``__init__`` closure. Middlewares read fields back via
-``runtime.context.<field>``; tools read them via ``runtime.context``.
-
-This object is read inside both ``KnowledgePriorityMiddleware`` (for
-``mentioned_document_ids``) and any future middleware that needs
-per-request state without invalidating the compiled-agent cache.
-"""
-
-from __future__ import annotations
-
-from dataclasses import dataclass, field
-from typing import TypedDict
-
-
-class FileOperationContractState(TypedDict):
-    intent: str
-    confidence: float
-    suggested_path: str
-    timestamp: str
-    turn_id: str
-
-
-@dataclass
-class SurfSenseContextSchema:
-    """
-    Per-invocation context for the SurfSense deep agent.
-
-    Defaults are chosen so the dataclass can be safely default-constructed
-    (LangGraph's ``Runtime.context`` itself defaults to ``None`` if no
-    context is supplied — see ``langgraph.runtime.Runtime``). All fields
-    are optional; consumers must None-check before reading.
-
-    Phase 1.5 fields:
-        search_space_id: Search space the request is scoped to.
-        mentioned_document_ids: KB documents the user @-mentioned this turn.
-            Read by ``KnowledgePriorityMiddleware`` to seed its priority
-            list. Stays out of the compiled-agent cache key — that's the
-            whole point of putting it here.
-        mentioned_folder_ids: KB folders the user @-mentioned this turn
-            (cloud filesystem mode). Surfaced as ``[USER-MENTIONED]``
-            entries in ``<priority_documents>`` so the agent prioritises
-            walking those folders with ``ls`` / ``find_documents``.
-        file_operation_contract: One-shot file operation contract for the
-            upcoming turn (reserved; not currently populated).
-        turn_id / request_id: Correlation IDs surfaced by the streaming
-            task; populated for telemetry.
-
-    Phase 2 will extend with: thread_id, user_id, visibility,
-    filesystem_mode, anon_session_id, available_connectors,
-    available_document_types, created_by_id (everything currently captured
-    by middleware ``__init__`` closures).
-    """
-
-    search_space_id: int | None = None
-    mentioned_document_ids: list[int] = field(default_factory=list)
-    mentioned_folder_ids: list[int] = field(default_factory=list)
-    mentioned_connector_ids: list[int] = field(default_factory=list)
-    mentioned_connectors: list[dict[str, object]] = field(default_factory=list)
-    file_operation_contract: FileOperationContractState | None = None
-    turn_id: str | None = None
-    request_id: str | None = None