Web-AI-SDK 0.5: Writer, Rewriter, Proofreader and Prompt API improvements

I've been exploring with the browser's Built-in AI APIs and built web-ai-sdk to make them easier to compose in applications.

Version 0.5 adds support for Writer, Rewriter and Proofreader, plus improvements to the Prompt API wrapper for on-device agents, including session cloning, tool passthrough and context introspection.

Feedback/PRs is welcome.