Fix setup wizard: mandatory step-by-step questionnaire

[iMark21]

Summary

• Rewrites .claude/commands/setup-screenshots.md with a gather-first, execute-later pattern
• Adds 4 mandatory stops that require user input before ANY files are created:
  1. Prerequisites check — CLI installed? UITests target? fastlane dir?
  2. Project analysis & screens — locales, brand colors, proposed screens for user confirmation
  3. Design template — user picks from 6 templates (this was being skipped)
  4. Confirmation checklist — full summary of what will be created, explicit "Ready?" prompt
• Execution phase only starts after all 4 stops are answered

Context

External contributor reported the wizard skipped the template selection step, generating all screenshots with a default style. Cost ~40 min of regeneration. The old "STOP and ask" instruction at step 2.5 was buried mid-flow and AIs didn't always respect it.

Test plan

• Copy updated setup-screenshots.md to a test project's .claude/commands/
• Run /setup-screenshots — verify it asks prerequisites before anything else
• Verify it shows project analysis and waits for confirmation
• Verify it asks for template choice and does NOT continue without it
• Verify it shows final checklist before creating any files
• Verify execution phase creates correct files after all confirmations