align migrate-repo to the ad-hoc model in .github (#576) (#585)
Epic vergil-project/.github#85, Task 9. Rework migrate-repo's ad-hoc handling for the new model: the ad-hoc bucket is the repo's 'Epic (ad hoc): ' in the org .github, ensured via 'vrg-adhoc-epic ensure --repo ' (was: create Epic (standing): Ad-hoc maintenance in the member repo via raw vrg-gh). The disposition renames standing->ad-hoc, and ad-hoc tasks native-link under the .github ad-hoc epic (--epic /.github#). Description, overview, setup, disposition table, batch order, linking, and report wording all updated. Last plugin-skill task of the epic.