fix: set git user config instead of --author flag

[wadefletch]

Summary

• Replace --author flag on git commit with git config user.name and git config user.email set before committing
• The --author flag only sets the author identity, not the committer identity. On GitHub Actions runners where no default git identity is configured, this causes fatal: empty ident name because git still has no committer to use.
• Setting user.name and user.email via git config configures both author and committer, resolving the error.

Test plan

• Deploy to a repo and trigger a fixup run on a GitHub Actions runner to confirm commits succeed without the fatal: empty ident name error