feat(coderd/database): add chat_context_resources table (#26430)

Adds chat_context_resources: a per-chat pinned copy of the agent context
resources a chat is hydrated against. The agent-side table
(workspace_agent_context_resources) is last-writer-wins with no history,
so a chat copies its resources at hydration/refresh to keep a stable view
while the agent drifts.

Schema foundation only (no queries/dbauthz/prepareGeneration/SDK yet).
chat_id FK ON DELETE CASCADE for cleanup parity; no agent FK so the pin
survives agent replacement; PK (chat_id, source); reuses the 000522 enum
types.

coderd/database/queries/chats.sql

@@ -2551,8 +2551,7 @@ WHERE id = @id::uuid;
 -- name: DeleteOldChats :execrows
 -- Deletes chats that have been archived for longer than the given
 -- threshold. Active (non-archived) chats are never deleted.
--- Related chat_messages, chat_diff_statuses, and
--- chat_queued_messages are removed via ON DELETE CASCADE.
+-- All chat-scoped child tables are removed via ON DELETE CASCADE.
 -- Parent/root references on child chats are SET NULL.
 WITH deletable AS (
     SELECT id