fix(dispatch): sort local dispatch candidates for stable output

Trail review: the trailer fallback pass ranges over a map (reachableCheckpointIDs), and Go randomizes map iteration order, so candidates were appended in a non-deterministic order. Nothing downstream re-sorts, so re-running dispatch --local over the same window could emit bullets in a different order — and thus a different LLM-authored summary — each time.

Sort the collected candidates most-recent-first (tiebreak by checkpoint ID) before returning, so dispatch output is stable across runs.