Implement GitHub agent definition loader

[bgervin]

PRD: GitHub Agent Definition Loader

Objective

Implement a module to fetch agent definitions from GitHub, supporting both Typespec and JSON formats. Must support public and private repos, using appropriate authentication if needed.

Tasks

• Accept GitHub repo and path for agent definition (Typespec or JSON).
• If Typespec, compile to JSON as above.
• Support authentication for private repos (token, managed identity, etc.).
• Parse JSON using Manifest library.
• Handle rate limits, network errors, and auth failures robustly.
• Add tests for all major code paths including auth scenarios.

Acceptance Criteria

• Loader accepts repo/path and loads both Typespec and JSON agent definitions.
• Auth options are documented and tested.
• Errors are handled gracefully.