[skills] Sync ai-sdk with current docs

[vercel-gh-bot-4]

• validate rule for stream\.write\( (overlay.yaml): the stale message "In createUIMessageStream, use stream.writer.write() not stream.write()" points agents at a stream.writer property that does not exist. createUIMessageStream passes the writer to execute({ writer }), and you write chunks with writer.write(). Corrected message: "In createUIMessageStream, use writer.write() on the writer passed to execute({ writer }), not stream.write() — the stream itself is not writable". Source: https://ai-sdk.dev/docs/reference/ai-sdk-ui/create-ui-message-stream (no last_updated). Quote: "execute ((options: { writer: UIMessageStreamWriterWithOutcome }) => Promise<void> | void): A function that receives a writer instance and can use it to write UI message chunks to the stream." The page's example also uses async execute({ writer }) { ... writer.write({ type: 'start' });.

Verification

run_plugin_checks passed every step: build (0 updated, 7 unchanged), typecheck, validate (PASSED), test (1079 pass, 0 fail). The build regenerated skills/ai-sdk/SKILL.md from overlay.yaml.

Test updates

None.

Not changed

• upstream/SKILL.md (in sync with vercel/ai@5028fa4) still tells agents to search via https://ai-sdk.dev/api/search-docs?q=your_query. That endpoint returns 404 even with a real query. It comes from the upstream source, so the fix belongs in vercel/ai skills/use-ai-sdk.
• The agent.generateText() / agent.streamText() rules say these were "renamed in AI SDK v6". The AI SDK 5 Experimental_Agent example in the 6.0 migration guide already uses agent.generate(), so I couldn't confirm the version. The recommended replacement is correct, so I left the rules alone.
• The experimental_generateImage rule doesn't mention that AI SDK 7 removed the export in favor of generateImage. Its recommendation still works, so I left it alone.
• metadata.docs sdk.vercel.ai URLs redirect to ai-sdk.dev pages for the same product, so they are not drift.

Opened automatically by the vercel-plugin drift agent. Review before merge.