feat: show GitHub username after PAT validation (#1004)

[alpurkan17]

Summary

• Fetch and display GitHub username (/user endpoint) after PAT validation
• Show Token for @username: valid instead of just Token: valid

Root Cause

After validate_pat_token() confirms a token is valid, it returns only boolean success. The miner doesn't know WHICH GitHub account the token belongs to — important when multiple tokens are configured.

Impact

Miners with multiple PATs can't verify which token maps to which GitHub account. Debugging token issues requires trial-and-error.

Why

Miners running multiple accounts need clear token-to-account mapping

Related Issues

Fixes #1004

Test plan

• ruff check
• ruff format --check
• pyright
• pytest tests/utils/test_github_api_tools.py::test_pat_validation_shows_username -q

Live verification

• gitt miner validate-pat now shows Token for @alpurkan17: valid

How to verify

1. Run gitt miner validate-pat with a valid PAT
2. Verify output shows @your-username in the output

Edge cases considered

• Token belongs to an organization (not a user) — handle gracefully
• Rate-limited /user endpoint — show fallback message
• Token with insufficient scope for /user — show partial info

Post-merge verification

• Confirm miner output shows correct GitHub usernames
• Check multi-token configs show distinct usernames per token

[anderdc]

Duplicate of #1005 which has been merged. Closing.