Document PR-only workflow

[RedBeret]

Summary

• add repository-level agent instructions requiring small topic branches and pull requests
• document the PR-only policy in contributing docs and the publication checklist
• keep direct main pushes reserved for the initial publication only

Validation

• bash scripts/publish-check.sh

Repository settings

Branch protection is now enabled on main with:

• pull requests required before merge
• smoke-test required and strict
• admin enforcement enabled
• force pushes disabled
• branch deletion disabled