Prefix pr-status replies with :robot: emoji (#90943)
### What?
Prefixes replies posted by `scripts/pr-status.js` with the `:robot:` (🤖) emoji.
### Why?
When the pr-status script replies to PR review threads, there's no visual indicator that the reply was generated by AI rather than a human. Adding the robot emoji makes it immediately clear.
### How?
One-line change in the `replyToThread()` function in `scripts/pr-status.js` that prepends `:robot: ` to the reply body before posting it via the GitHub GraphQL API.
