Add --baseline mode: track trust score regression across commits

[rudra496]

Add a baseline mode that establishes a trust score baseline and alerts on regressions.

Usage: aitrust validate src/ --baseline .aitrust-baseline.json then aitrust validate src/ --compare-baseline .aitrust-baseline.json

Features: per-file score tracking, regression detection with configurable threshold, CI exit code 2 for regressions, improvement reporting.

See CONTRIBUTING.md for development setup. Advanced difficulty — requires multi-run state management.