fix(web-chat): let an operator choose the chat agent, and stop a raw web_fetch buying an LLM summary of markup - #6586

On November 2, 2026, GitHub will restrict `pull_request_target` on public repositories by default. To continue allowing the event trigger, configure an Actions policy. Learn more: https://gh.io/securely-using-pull_request_target#default-policy-for-pull_request_target