Resolve UNKNOWN mergeable status in auto-triage via REST API (#63317)

GitHub's GraphQL API computes mergeability lazily and often returns
UNKNOWN. Add a phase that queries the REST API (which triggers the
computation) for PRs with unknown status, with one retry after a
short delay for cases where GitHub hasn't finished computing yet.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>