Add --upload for generated HTML in GitHub repositories

[asafmor]

Pageleaf can now publish its generated HTML to an existing GitHub repository with --upload [owner/]repo. For example, pageleaf ./docs --upload pages commits docs.html to the root of the signed-in user's pages repository on its default branch, replacing an existing file. An explicit owner such as --upload my-org/pages selects that account or organization.

The upload uses gh for GitHub identity and repository access, and Git with a temporary bare repository for the commit and normal push. It preserves unrelated files, supports empty repositories, skips identical content, and uses the signed-in user's GitHub noreply identity. Remote replacement is automatic; --force retains its existing local-output meaning.

Failures explain the next steps for missing tools, authentication, repository access, directory collisions, local I/O, network issues, and rejected pushes. The generated HTML remains available after a failed upload. Missing repositories are not created automatically, and GitHub Pages is not configured.

Validation:

• All 28 local tests pass, including real local Git insert/overwrite/no-op, empty repository, symlink, directory collision, concurrent push, and failure scenarios.
• Live CLI insert, overwrite, and unchanged-content checks passed against asafmor/pages using both owner forms. Uploaded bytes matched the generated file and unrelated entries remained unchanged. The temporary HTML was removed with a follow-up Git commit, restoring the original root tree.
• Package dry-run includes both new library modules.
• Added PR CI for Node 20, 22, and 24 on Linux, plus Node 24 on Windows and macOS, including package dry-run checks.