add vrg-adhoc-migrate to relocate standing epics into .github (#2128) (#2173)
Phase 2 (epic #85). One-shot migration: find open epic+standing issues across the org, and for each — ensure its ad-hoc replacement (Epic (ad hoc): ) in /.github, re-link the standing epic's OPEN children under it (unlink from standing, link to ad-hoc), and close the standing epic with a pointer comment. Closed children stay with the retired epic for history. Dry-run by default; --apply is human-gated (refused in agent sessions, mirroring vrg-epic-audit --close). Idempotent: a closed standing epic drops out of the open-epic search. New lib adhoc_migrate + vrg-adhoc-migrate CLI; 100% covered.