docs: Update documentation for AtomizerSpec v2.0 unified configuration
Phase 4 documentation updates:
- CANVAS.md: Add AtomizerSpec v2.0 schema, custom extractors section,
spec REST API endpoints, and updated file structure
- DASHBOARD.md: Add V3.1 release notes, AtomizerSpec API endpoints,
and unified configuration features
- CLAUDE.md: Add complete AtomizerSpec v2.0 section with code examples,
MCP tools reference, API endpoints, and updated directory structure
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>