Drain the milestone-merge GitHub outbox (every 10 minutes)

HO-15660. Runs one bounded drain of the milestone-merge GitHub outbox (roadmap_sync_intents): at most 10 intents, two at a time, each GitHub exchange capped at 10 s, legacy direct-send recovery first. While any intent of a merge is unsynced, every edit of its survivor is refused (MILESTONE_SYNC_UNRESOLVED). A GitHub refusal is a count, retried with backoff on a later run; ok is false only when the drain itself threw. Protected by CRON_SECRET via the x-cron-secret header. Scheduled by the milestone-merge-sync stage of the embedding-drain schedule in .slipstream.json.