MemoStem plugin v0.6.1

Updates the public MemoStem skill so an explicit unnamed invocation keeps the existing capture flow, then falls back to recent conversation context only when the current conversation has no eligible candidate and the host actually provides that context. It reports unavailable context honestly and never transfers source transcripts.\n\nValidation: public plugin checks (44 tests), submission-readiness preflight, private MemoStem compatibility, and exact-main CI passed. Provider review and directory publication remain separate.