[log] Add debug logging to GitHub token/URL resolution in envutil

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Summary

Adds debug logging to internal/envutil/github.go to improve visibility into GitHub token and API URL resolution during troubleshooting.

Changes

File modified: internal/envutil/github.go

• Added logger import and var logGitHub = logger.New("envutil:github")
• LookupGitHubToken: 3 log calls
  • Entry log showing how many candidate env vars are checked
  • Log which env var key supplied the token (key name only, not the value — secure)
  • Log when no token is found in any candidate var
• LookupGitHubAPIURL: 2 log calls
  • Log URL when GITHUB_API_URL env var is used
  • Log URL when falling back to the default

Why This Is Useful

LookupGitHubToken is called during auth setup and it's often unclear in production which env var is providing the token (or if one is missing). These logs make that immediately visible when DEBUG=envutil:* is set, without leaking the token value.

Similarly, LookupGitHubAPIURL is called during server startup and knowing whether the URL came from env or defaults helps diagnose routing issues.

Testing

Enable debug logging to see output:

DEBUG=envutil:* ./awmg --config config.toml
```

Expected output:
```
envutil:github Looking up GitHub token: checking 4 candidate env vars
envutil:github GitHub token found: source=GITHUB_TOKEN
envutil:github GitHub API URL using default: https://api.github.com

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