feat: add schema drift detection (#50)

[Astro-Han]

Description

Add runtime schema drift detection that warns users when command output fields are suspiciously empty, indicating upstream API structure changes.

Related issue: #50

How it works

After a command executes and before output renders, detectDrift() checks each declared column's empty-value ratio (null, undefined, ''). If any column is >= 80% empty across valid object rows (minimum 3), a yellow warning is printed to stderr:

⚠ Schema drift detected (bilibili/hot):
  • field 'author' — empty in 15/15 rows (100%)
  • field 'danmaku' — empty in 12/15 rows (80%)

New src/drift.ts module with detectDrift() and formatDriftWarning() — standalone, no side effects. Integrated into cli.ts between command execution and output rendering (warning only, no exit code change).

Closes #50 (core detection layer — CI cron and LLM repair interface deferred to follow-up)

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Screenshots / Output

$ npm run dev -- hackernews top --limit 3

  hackernews/top
┌──────┬──────────────────────────────────────┬───────┬─────────────┬──────────┐
│ Rank │ Title                                │ Score │ Author      │ Comments │
├──────┼──────────────────────────────────────┼───────┼─────────────┼──────────┤
│ 1    │ Astral to Join OpenAI                │ 307   │ ibraheemdev │ 136      │
├──────┼──────────────────────────────────────┼───────┼─────────────┼──────────┤
│ 2    │ OpenBSD: PF queues break 4 Gbps      │ 32    │ defrost     │ 8        │
├──────┼──────────────────────────────────────┼───────┼─────────────┼──────────┤
│ 3    │ Juggalo Makeup Blocks Facial Recog...│ 61    │ speckx      │ 18       │
└──────┴──────────────────────────────────────┴───────┴─────────────┴──────────┘
3 items · 9.7s · hackernews/top

No drift warning (all fields populated) — expected behavior for healthy adapters.

Test results

$ npx vitest run src/drift.test.ts
 ✓ 14 tests passed (drift.test.ts)

$ npm run typecheck
 tsc --noEmit (no errors)

$ npm run build
 ✅ Manifest compiled: 213 entries