mpai v0.4.6 — privacy-safe alpha receipts

Adds mpai alpha-receipt: a local, mode-0600, review-before-sharing activation and reliability receipt for the first-10-team cohort. It sends nothing automatically and excludes prompts, transcripts, names, task identifiers, paths, credentials, network addresses, and event timestamps. Also includes the v0.4.5 peer-unreachable recovery hardening and all prior public-alpha behavior.