Add deploy guard to validate data before build proceeds

Prevents deploying degraded data by checking exported JSON files:
- Minimum book count per year (default 100)
- Regression check: fails if count drops >30% from committed data
- Required fields validation (id, title, author, releaseDate)
- Sanity checks: no duplicate IDs, valid dates, no empty strings

Runs automatically as part of `npm run build`. Use --skip-guard to
override when intentionally resetting data.

Closes #31

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>