Preserve commit counts in GitHub project fallback

[andrewkernel]

Summary

• preserve author and total commit counts when GitHub project selection falls back
• keep fallback project JSON aligned with the primary serialization path
• add a focused unittest that forces the fallback path

Verification

• python -m unittest tests.github_projects_fallback_test
• python -m py_compile github.py tests/github_projects_fallback_test.py