docs(ai-chat): guide for migrating an AI SDK route handler to chat.agent

Developers arriving with a working Vercel AI SDK chat app had no page telling
them which parts of it survive the move to chat.agent and which parts get
deleted. This walks through it with before/after code: the streamText call,
model config, tools, and useChat stay; the route handler, its persistence glue,
and any resumable-stream setup go; the agent task, two server actions, and
useTriggerChatTransport are new.

Also includes a copy-pasteable prompt that points a coding agent at the live
docs and the version-pinned SDK skills, so the migration can be handed off.

Author: matt-aitken