Consolidate CLAUDE.md to reference socket-registry for shared standards

Reduced file size from 35,682 to 6,714 bytes (81.2% reduction) by:
- Referencing socket-registry/CLAUDE.md as canonical source for shared patterns
- Retaining purl-specific patterns: PurlError handling, spec compliance, performance requirements
- Maintaining index signature patterns and bracket notation rules
- Keeping TypeScript exactOptionalPropertyTypes guidance

Author: jdalton