Function drain_queue 

pub async fn drain_queue<R, Fut>(
    repo_root: &Path,
    once: bool,
    run_mission: R,
) -> Result<DrainReport>
where
    R: Fn(String) -> Fut,
    Fut: Future<Output = Result<i32>>,

Drain the per-repo queue one mission at a time. Recover dead claims once up front; then atomically claim the front entry with the repo-wide busy guard; on Busy either return (once) or sleep 5s and retry; on a lost claim race, retry after a brief sleep; on Claimed, run backend readiness under the claim (park → finish_claim; rate-limit → release + rotate/delay; ok → run), skip a ticket-born entry whose blocker failed, otherwise write ticket Running, invoke the injected run_mission, then finish/release the claim, write the terminal ticket state, and honor once.

Readiness is probed after claim so a sibling drain cannot race a peek

• queue::remove into marking a live run’s ticket Failed.

Does NOT do checkout restoration or event printing — those stay with the caller, which is exactly why run_mission is injected rather than run inside this core: the CLI keeps its live event tail, while a headless caller (REST, Slack) can drive the same loop with no terminal attached.