Add tests for GitHub Action mode

[pouryawJs]

Summary

The current test coverage focuses on core deterministic logic. We should add tests around the GitHub Action entrypoint behavior.

Why this matters

GitHub Action mode is one of the main ways users will adopt Maintainer Agent Kit, so we should verify its behavior as the project grows.

Suggested coverage

• Reads config path from action input
• Respects dry-run
• Handles missing config file gracefully
• Handles unsupported mode gracefully
• Logs matched labels
• Does not call label mutation in dry-run mode

Acceptance criteria

• Add tests for src/action.ts or extracted action runner logic
• Mock GitHub Action inputs
• Mock GitHub API calls where needed
• Keep tests deterministic