Improved metric formula, adding MI support

What's Changed

• Add comprehensive tests for boolean ops, match/case, and MI by @zhanglpg in #18
• Add improvement roadmap with prioritized next steps by @zhanglpg in #19
• Phase 1: fix compare_refs bug, update README, add --output flag and pre-commit hook by @zhanglpg in #20
• Update roadmap: mark Phase 1 and Phase 2 as complete by @zhanglpg in #21
• Implement Phase 3: class metrics, Halstead, caching, HTML/SARIF output, plugins by @zhanglpg in #22
• Extract models.py to reduce coupling — NCS 7.01 → 6.80 by @zhanglpg in #23
• Replace binary hotspot ratio with weighted severity in NCS formula by @zhanglpg in #24
• Add /refactor-complexity skill for metric-driven refactoring by @zhanglpg in #25
• Add prerequisites section to refactor-complexity skill by @zhanglpg in #26

Full Changelog: v1.3...v1.4