Preview deployments now available for pull requests

Reviewers no longer need to clone PRs and run builds locally to validate changes. A new automated workflow detects pull request events, builds the CLI from the PR's source code, runs the analyze and build pipeline, then deploys to a unique Vercel preview URL. That URL gets posted as a single sticky comment on the PR — updated on each push rather than spawning new comments. Fork PRs are skipped automatically since secrets aren't available in that context, and in-flight builds are cancelled when newer commits arrive, keeping previews current.