docs(ai-chat): keep anchor stubs for moved backend sections

Inbound deep links to the sections that moved to the Custom agents
page (#chat-createsession, #chat-customagent, #raw-task-with-primitives)
now land on the router stub instead of the top of the Backend page.
Path-level redirects cannot cover this since fragments never reach the
server.

Author: ericallam

docs/ai-chat/backend.mdx

@@ -803,6 +803,11 @@ export const manualChat = task({
 
 ---
 
+{/* Anchor stubs for inbound deep links to the sections that moved to /ai-chat/custom-agents. */}
+<a id="chat-createsession" />
+<a id="chat-customagent" />
+<a id="raw-task-with-primitives" />
+
 ## Custom agents
 
 Both lower levels — `chat.createSession()` (managed turn iterator, your turn body) and `chat.customAgent()` with raw primitives (hand-rolled loop, full stream-conversion control) — are covered together on the Custom agents page, including the `ChatTurn` surface, the continuation-seeding pattern, and the hand-rolled-loop checklist: