Checkout action updated to v6 across workflows

GitHub Actions workflows have been updated to use the latest checkout action. The .github/workflows/ directory now runs actions/checkout@v6 instead of v4 across all four workflow files — ci.yml, publish.yaml, release-please.yml, and release.yml. This upgrade brings bug fixes for tag handling, worktree support for persisted credentials, and updated Node.js runtime support. CI pipelines benefit from a more reliable checkout process without any functional changes to how the workflows operate.