Add GitHub OAuth bootstrap admin env sync

[cbusillo]

Summary

• request verified GitHub email scope and use verified private emails for bootstrap admin access
• add a narrow self-deploy OAuth env sync path for the known Launchplane GitHub OAuth env keys
• document the OAuth env contract and ignore local .env files

Verification

• uv run python -m unittest
• npx pnpm@10.10.0 --dir frontend validate
• git diff --check
• docker build -t launchplane-oauth-bootstrap-admin-test .
• actionlint .github/workflows/deploy-launchplane.yml