Skip auth prompt in install.sh if already authenticated

[ivpusic]

Summary

• Before prompting the user to authenticate during install, check if they're already authenticated by running dune usage
• If the check succeeds, print "Already authenticated with Dune." and skip the auth prompt entirely
• Avoids unnecessary re-authentication on reinstalls/upgrades

[cursor]

PR Summary

Low Risk
Low risk: small shell-script change limited to post-install UX, with the only behavior change being conditional suppression of the auth prompt based on dune usage exit status.

Overview
Updates install.sh post-install flow to first probe authentication by running dune usage; if it succeeds, the script logs Already authenticated with Dune. and skips the interactive dune auth prompt.

Otherwise, the existing authentication prompt and dune auth invocation remain unchanged.

^{Written by Cursor Bugbot for commit a02c499. Configure here.}

[va3093]

btw, can you sync with platform on what merge rules we need to have.

You shouldn't be able to merge this without a review.