Add PR intake loop runbook

[kamalsrini]

Summary

• Add docs/pr-intake-loop.md as the reusable /loop procedure for incoming GitHub PRs.
• Document Linear ticket creation/linking, legitimacy review gates, local verification, GitHub checks, merge handling, and change-request handling.
• Link the runbook from README and CONTRIBUTING.

Linear: UNI-27

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