Add conditional messaging for onboarding on blank template

[JamesW1]

Changes

Do not merge before: directus-labs/starters#84 ⚠️

Updates the init success message to be conditional:

• Shows admin credentials when ADMIN_EMAIL and ADMIN_PASSWORD are provided
• Shows onboarding message when they're not provided

Context

Linked to recent starters update (directus-labs/starters#84), the 'blank' template now uses Directus's onboarding flow instead of pre-configured admin accounts. This PR makes the CLI messaging aware of that pattern.

Before

Always showed: "You can login with email/password"

After

• Templates with credentials: Shows login instructions
• Templates without credentials: Shows onboarding instructions

Testing

• Tested with blank template (no credentials) → shows onboarding message ✅
• Tested with CMS template (has credentials) → shows login message ✅