Streamlining Release Automation

Oppia Android's release process currently relies on a 17-page manual document that release coordinators follow step-by-step, covering everything from cutting branches and building binaries to signing, deploying to Play Console, and writing changelogs. This is slow, error-prone, and has caused QA testers to miss builds due to Play Store sync issues. This project replaces that manual process with six GitHub Actions workflows backed by Kotlin scripts that handle building, Cloud KMS signing, Play Console deployment, Firebase App Distribution for QA, LLM-powered changelog generation, and automated weekly alpha releases. All scripts are designed with interface-based abstractions (CloudSigner, PlayConsoleClient, VertexAiClient, GitHubClient) so they can be thoroughly unit-tested with fakes, requiring no real GCP credentials in tests. Security is handled through Workload Identity Federation (keyless auth) and Cloud KMS HSM-backed signing, ensuring the private key never leaves Google's hardware. Deliverables: 6 new Kotlin scripts with full test coverage, 6 GitHub Actions workflows, updated wiki documentation (playbook + manual fallback reference), and an end-to-end demo of the complete release cycle running through CI.