feat: upgrade gh-pages from 3.1.0 to 6.3.0

Major version upgrade with no breaking changes detected.
All 381 tests pass without modification.

Key findings:
- gh-pages/lib/git internal API unchanged
- Error message format compatible
- Git command behavior unchanged
- Promise handling already defensive (works with v3 and v6)

Closes #186

Author: JohannesHoppe