feat: add whitewind support to backfill-pds script + fix dep URLs
- backfill-pds now handles com.whtwnd.blog.entry collection
- extracts markdown content from whitewind's content field
- sets platform to "whitewind", skips visibility:"author" entries
- prefers publishedAt over createdAt for date extraction
- update tangled.sh URLs to tangled.org in build.zig.zon
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>