Add skill install and dune auth to the install.sh script

[ivpusic]

Currently there are separate steps, we are not consolidating that into single script.

[cursor]

PR Summary

Low Risk
Changes are confined to the installer’s post-install UX and optional commands; main download/verify/install logic is unchanged, with minimal risk beyond potentially surprising interactive prompts.

Overview
Enhances install.sh to run a post-install interactive setup when a TTY is available, prompting the user to (optionally) install the Dune AI coding skill via npx skills add duneanalytics/skills and to authenticate by running dune auth.

For non-interactive installs (no /dev/tty), the script now prints success plus follow-up tips instead of running prompts, and it replaces the previous single “Installed …” log line with this new flow.

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