chore: canonicalize fork maintenance on main

[aeei]

Summary

• make main the CI push target for the maintained fork
• document upstream ownership, tag-merge policy, validation, and immutable artifact gates
• establish the current v5.1.2 upstream baseline

Validation

• focused Prettier
• actionlint when available
• git diff --check

Repository setting follow-up

After merge:

1. archive the stale shadcn-theme head with a tag
2. set GitHub default branch to main
3. protect main with required validate checks
4. delete stale shadcn-theme