Revert public PR intake loop runbook

[kamalsrini]

Summary

• Revert PR Add PR intake loop runbook #2757 so the internal PR intake loop is no longer published in the public repository.
• Remove docs/pr-intake-loop.md and its README/CONTRIBUTING links.
• The reusable workflow is being kept as a private local Codex skill instead.

Verification

• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• ruby scripts/test_skill_fixtures.rb
• ruby scripts/generate_quality_scorecard.rb --check
• ruby scripts/validate_framework_registry.rb
• ruby scripts/validate_framework_registry.rb --stale --max-age-days 365 --as-of 2026-06-18
• ruby scripts/validate_codeowners.rb
• git diff --check HEAD~1 HEAD