Document the GCP Buzz pilot and secret naming

[shyamdurai]

Summary

• document the current single-VM Docker Compose deployment on GCP
• add the pinned bootstrap and Buzz_ Secret Manager IDs
• preserve the Tauri origins required by the desktop invite flow
• clarify that the pilot uses local MinIO, not managed Google Cloud Storage

Verification

• bash -n deploy/gcp/bootstrap.sh
• git diff --check
• live GCP health was verified separately after the PostgreSQL/Redis rotation

No secret values are included. Unrelated local files were not staged.