GitHub Actions checkout upgraded to v6

CI tooling has been brought up to date. Four GitHub Actions workflows — CI, publish, release-please, and release — now use actions/checkout@v6 instead of v4. The checkout action handles cloning repositories during automated runs, so staying current means fewer surprises when GitHub updates runner environments. The v6 release includes Node.js 24 support, better credential management (storing tokens in a separate file rather than git config), and fixes for tag handling and worktree scenarios.