Convert to read-only mirror (#35)

## Summary

- Remove PR-related workflows and validation (provenance checks, schema
validation, dependency review, yamllint)
- Add read-only mirror notice to README
- All validation now happens internally before publishing
- This repo receives direct pushes from the auto-publish pipeline — no
more PRs needed

## Test plan

- [x] README updated with mirror notice
- [ ] Verify direct push works end-to-end after merge

Author: saada