Split monolithic action.ts into smaller modules #461
Description
The core orchestration file (src/github/action.ts) is ~900 lines and handles cloning, prompt building, Codex/OpenAI invocation, diff detection, PR creation, comment updates, and error handling. Break it into cohesive submodules (e.g. prompt-builder, diff-scanner, comment-manager) to improve readability, testability, and maintainability.