Harden and deploy the deterministic GitHub App on Cloud Run

[kunaldhongade]

Closes the production gaps in the optional deterministic GitHub App and deploys it to Google Cloud Run.

Scope:

• Harden webhook error handling, comment pagination, concurrency, and checkout limits.
• Add container and Cloud Run deployment assets.
• Add deployment and GitHub App operations documentation.
• Validate the App in staging before broad installation.

The App remains deterministic: no project commands, LLM calls, deployments, telemetry, or hosted CodeDecay workflows.