* fix(ci): poll mergeable until resolved in conflict detector
GitHub computes pr.mergeable asynchronously and returns null ('still
computing') for minutes after a push. The detector treated null as
'leave as-is' and never re-checked, so conflicting PRs were never
labeled has-conflicts.
Add a retry loop (getMergeable) that polls until GitHub resolves the
merge state (or times out), so conflicts are reliably labeled.
* fix(ci): poll mergeable, dedupe concurrency, drop checkout, batch PRs
Refine merge-conflict-detector after review:
- concurrency group uses github.ref (stable for both PR + push
triggers) so cancel-in-progress actually fires on the bulk path.
- drop actions/checkout + contents:read (pure API, no FS needed).
- getMergeable returns the full PR object (no re-fetch in
checkAndLabel).
- swallow 422 on createLabel and 404 on removeLabel (race-safe).
- batch open PRs in chunks of 5 via Promise.all to bound wall-clock.
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Co-authored-by: M57 <hy4ri@users.noreply.github.com>
