feat(tooling): add lexicon validation script

Add convenient shell script for validating all lexicon schemas and test data.

**Added:**
- scripts/validate-schemas.sh - Wrapper around cmd/validate-lexicon

**Usage:**
```bash
./scripts/validate-schemas.sh
```

**Features:**
- Validates all 58 lexicon schema files
- Validates cross-references between schemas
- Tests all lexicon test data files (15 valid, 11 invalid)
- Reports test coverage per record type

This script makes it easy to verify lexicon changes before committing,
addressing the PR review requirement for lexicon validation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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