Render a Claude Code SessionStart envelope for a lifecycle hook

T858. Takes the raw .contextq/config.json bytes plus a few git facts as application/x-www-form-urlencoded and returns the FINISHED Claude Code SessionStart envelope, which the hook prints verbatim. The caller builds and parses no JSON at all, so a plugin hook needs nothing on the machine but bash, curl and git. Answers 204 when the repo is disabled or the boot pack would be empty, so the hook prints nothing.