[WIP] Add custom agent for W3C style specification maintenance

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Create a custom agent to create and maintain W3C style specification, similarly to the MCP gateway in this repo.

• strongly inspired by the technical documentation writer in terms of technique to write. Borrow from that prompt for writing style.
• follows style and rigor of w3c specifications
• implements best practice to modify, version and changelog updates

This agent is agnostic of the project, not gh-aw specific.

Custom agent used: technical-doc-writer
AI technical documentation writer for GitHub Actions library using Astro Starlight and GitHub Docs voice

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