Add extensible Slop Cop Dev Note checks

[johnnygreco]

Summary

• add Slop Cop as a repository development tool for scoring configured editorial signals in Dev Notes
• provide built-in, declarative, local custom, and external-service rule extension points
• add bounded density-aware scoring, suppressions, audited manual overrides, JSON output, and a self-contained HTML report
• add historical calibration benchmarks and enforce their score ranges in candidate CI validation
• add read-only PR analysis and a separate trusted workflow for validating artifacts and updating a sticky PR comment
• require absolute imports in handwritten Python package code

CI rollout

This introducing PR cannot enforce its own default-branch workflow definition. After merge, configure a repository or organization ruleset that requires .github/workflows/slop-cop.yml for main, requires pull requests and current branches, protects both Slop Cop workflow paths, and restricts bypass permissions.

Validation

• pytest — 117 passed
• ruff check .
• ruff format --check .
• mypy src tests
• slop-cop validate-rules
• slop-cop benchmark --repository-root ../.. — six references passed
• full current Dev Note scan — pass at 95/80