idempotently ensure a repo's standing epic exists (create-if-missing) (#2084)
Standing epics are per-repo (every member repo and .github has its own 'Epic (standing): Ad-hoc maintenance', labelled epic+standing). epics.ensure_standing_epic(repo) now returns that epic, creating it if absent and reusing it otherwise — idempotently; several is still ambiguous and errors. resolve_epic_ref('standing') delegates to it, so vrg-issue-create --epic standing and vrg-epic-move --epic standing provision the standing epic on the fly. Adds vrg-standing-epic ensure --repo for callers (migrate-repo step 1) that must provision before linking pre-existing issues. Non-standing epic creation is unchanged (.github-only via vrg-epic-create). Ref vergil-project/vergil-tooling#2083, vergil-project/.github#40.