feat: add skill sync drift summary

[rogerchappel]

Summary

• add a pure helper for classifying skill sync drift
• add driftStatus and requiresReview to skill sync status items
• add aggregate needsReview, changed, and unknown counts for governance visibility

Verification

• git diff --check
• pnpm test src/lib/skill-sync-drift.test.ts
• pnpm exec eslint src/lib/skill-sync-drift.ts src/lib/skill-sync-drift.test.ts src/app/api/skills/sync-status/route.ts
• pnpm exec tsc --noEmit --pretty false
• push hook: pnpm typecheck && pnpm build

Risk

Low. This is additive read-only response data and does not sync skills, mutate runtime config, touch secrets, or change existing status semantics.