Add CLI troubleshooting instructions to STUDIO.md

[bcotrim]

Related issues

• Fixes STU-1483

How AI was used in this PR

AI was used to draft the debugging instructions and create the PR.

Proposed Changes

• Add a "studio command not found" troubleshooting entry to the Debugging section of STUDIO.md, instructing AI agents to guide users to enable the CLI toggle in Studio settings when the studio command is unavailable.

Testing Instructions

1. Build and run the app with npm start
2. Start a site so the updated STUDIO.md gets deployed to the site directory
3. Uninstall the CLI from Settings → Studio CLI for terminal
4. Open a new terminal at the site path and confirm studio returns "command not found"
5. Use an AI agent (e.g. Claude Code) in that site directory — it should pick up the new instructions and guide you to re-enable the CLI toggle

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[fredrikekelund]

To test this properly, I had to also run npm install to trigger the scripts/download-agent-skills.ts script. After doing that, this works just as expected 👍