release: add stable signed APK workflow

[faheemKamboh]

Summary

• add release signing configuration driven by GitHub Actions secrets
• build and upload a signed release APK artifact when signing secrets are configured
• bump Android metadata to v0.1.6 / versionCode 7
• update release checklist and implementation TODOs

Validation

• git diff --check
• workflow YAML parsed locally
• restricted permission scan still confirms SEND_SMS and no READ_SMS
• signing secrets are configured in GitHub Actions; CI should prove :app:testDebugUnitTest, :app:assembleDebug, and :app:assembleRelease