finish-feature

Complete epic with cleanup and archiving, or verify feature already closed. MUST BE USED when finishing epics/features. For epics: Verifies children closed, archives docs, caches to Serena, closes epic. For features/tasks/bugs: Verifies already closed (from PR creation), archives docs, caches to Serena. Non-epic issues must be closed at PR creation time (atomic merge pattern). Use when user says "I'm done with this epic", "finish the feature", "finish this epic", "archive this epic", or when user mentions epic completion, cleanup, archiving, feature finalization, or closing work.