Assign issues to g1t and @g1t in threads: g1t takes the work on, reacts 👀, sessions in the timeline #135

Shipped in e77b8207 (Deploy #342): assign an issue to g1t or say @g1t in an issue or pull request, g1t reacts 👀 and starts a session on the workspace's Claude as whoever asked, and sessions show in the conversation as cards. Reactions are in the API (POST /repos/{owner}/{name}/issues/{number}/reactions) and the site.