add ad-hoc epic model in .github, deprecate standing (#2122) (#2131)
Core Phase 1 change for epic #85. ensure_adhoc_epic locates/creates a repo's ad-hoc epic in /.github, titled 'Epic (ad hoc): ' and labelled epic+ad-hoc, disambiguated by title (many coexist, one per repo). resolve_epic_ref accepts the 'adhoc' sentinel with 'standing' kept as a deprecated alias; rollup treats ad-hoc OR standing as perpetual. ensure_standing_epic remains as a module-level alias, and the standing label stays (retired in Task 11). Backward-compatible: the 'standing' sentinel now resolves to the .github ad-hoc epic; existing per-repo standing epics are relocated by the Phase 2 migration.