feat: add schema drift detection (#50)

[Astro-Han]

Summary

• New src/drift.ts module with detectDrift() and formatDriftWarning() — standalone, no side effects
• Warns on stderr when command output columns have >= 80% empty values, indicating upstream API schema changes
• Integrated into main.ts between command execution and output rendering (warning only, no exit code change)
• 14 unit tests covering all edge cases (threshold boundaries, mixed empty types, non-object rows, minimum sample guard)

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

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%)

Test plan

• npx vitest run src/drift.test.ts — 14 tests pass
• npm run typecheck — no errors
• npm run dev -- hackernews top --limit 3 — normal output, no false positive