Add High Impact Files section to initial context showing which files
are most critical based on percentage of codebase that depends on them.
Changes:
- Add impactScore field to FileMeta (0-100)
- Add calculateImpactScore() helper function
- Update MetaAnalyzer to compute impact scores
- Add formatHighImpactFiles() to prompts.ts
- Add includeHighImpactFiles config option (default: true)
- 28 new tests (1826 total)