release: define GA readiness checklist

[flyingrobots]

GA Checklist (must all be green)

• All `--json` outputs schema-validated in CI
• Public API snapshot unchanged (or semver-major justified)
• Typed errors + stable exit codes everywhere
• Atomic writes proven under injected failures
• Doctor strict catches/refuses all known corruption classes
• Fresh-user tutorial validated by at least 2 non-authors
• CI policy gate docs + working example repo
• Performance baseline recorded and regression alarms active

Part of M9 — IRONCLAD GA

[flyingrobots]

Closing under the clarified planning model. A milestone-style GA checklist is no longer the primary control plane for Git Mind; Hills, Playbacks, and issue-level execution are. Specific readiness work should live in Hill-aligned issues rather than an abstract GA gate.