refactor: leverage gh-pages v6 native features

- Delegate CNAME and .nojekyll file creation to gh-pages v6+ (PR #533)
- Remove createCnameFile() and createNojekyllFile() - gh-pages handles this now
- Switch from callback-based to Promise-based gh-pages API (v5+ fix)
- Add real filesystem tests for CNAME/.nojekyll file creation verification
- Update all test mocks from callback to mockResolvedValue/mockRejectedValue
- Fix gh-pages.clean() test to properly test repo-specific cache deletion
- Update documentation baseline from v3.2.3 to v6.3.0

This commit completes the gh-pages v6 upgrade by using native features
instead of custom workarounds.

Author: JohannesHoppe

docs/README_contributors.md

@@ -272,7 +272,7 @@ The `getRemoteUrl` function in `src/engine/engine.prepare-options-helpers.ts` ca
    - If the internal API changed significantly: implement our own git remote discovery using `child_process.execSync('git config --get remote.{remote}.url')`
    - If gh-pages added a public API for this: switch to the public API
 
-**Current baseline:** Tests are pinned to gh-pages v3.2.3 behavior and error messages.
+**Current baseline:** Tests are pinned to gh-pages v6.3.0 behavior and error messages.
 
 ## Keeping track of all the forks