refactor: Remove redundant /api directory and consolidate to /docs/api

- Eliminated duplicate /api directory (not used by Vercel)
- Updated /docs/api/README.md to reflect single source of truth
- /docs/api/track-download-supabase.js already includes 'skill' type support
- Simplified maintenance workflow (no more file syncing needed)

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: davila7