Update M6 docs to reflect release.yml mirroring link/'s engine (auto version + changelog + tag + signed APK + Gitea release on merge to main), not a v* tag trigger. Note REGISTRY_USER/REGISTRY_TOKEN + the main-push requirement, and the derived monotonic versionCode. §9 status, §9 M6/M7 milestones, §12. Co-Authored-By: Claude <noreply@anthropic.com>