CLI: Only ask for admin permissions once when changing domain

[bcotrim]

Related issues

• Fixes STU-1077

Proposed Changes

• Add updateDomainInHosts() function to cli/lib/hosts-file.ts that handles removing the old domain and adding the new domain in a single hosts file write operation (single admin privilege request)
• Add skipHostsUpdate option to setupCustomDomain() in cli/lib/site-utils.ts to avoid duplicate hosts file updates
• Refactor cli/commands/site/set-domain.ts to use the new combined operation

Testing Instructions

1. Create a site with no custom domain
2. Run studio site set-domain test.local --path <site-path>
   • Verify you're only prompted for admin password once
   • Verify the domain is added to /etc/hosts
3. Run studio site set-domain test2.local --path <site-path> to change the domain
   • Verify you're only prompted for admin password once (previously this asked twice)
   • Verify test.local is removed and test2.local is added to /etc/hosts
4. Run unit tests: npm test -- cli/commands/site/tests/set-domain.test.ts

Pre-merge Checklist

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