Codebase cleanup: schema fixes, CI validation, and documentation improvements

[gvonness-apolitical]

Summary

• Schema fixes: Add missing allOf conditions for horizontalRule and break block types; fix tableCell children to require blocks only (resolving oneOf validation ambiguity); fix cross-schema references to use full URLs
• CI/CD: Add GitHub Action for automated schema compilation and example document validation
• Documentation: Create comprehensive example document demonstrating all block types; update spec for tableCell changes; add strategic insights to design-decisions.md; archive strategy notes

Details

Schema Fixes

1. horizontalRule/break validation (content.schema.json): These block types were defined in $defs but missing from the allOf conditional chain, causing documents using them to not validate correctly.

2. tableCell children ambiguity (content.schema.json): The oneOf: [textNode, block] pattern was ambiguous because both schemas have a type field. Changed to block-only (consistent with listItem). Updated spec documentation and examples accordingly.

3. Cross-schema references (annotations.schema.json, phantoms.schema.json): Changed relative references like anchor.schema.json#/$defs/contentAnchor to full URLs https://codex.document/schemas/anchor.schema.json#/$defs/contentAnchor for proper JSON Schema resolution.

CI/CD

New GitHub Action (.github/workflows/validate-schemas.yml) that:

• Compiles all schemas to verify they're valid JSON Schema
• Validates example documents against their schemas
• Handles cross-schema references properly

Documentation

• Comprehensive example: New examples/comprehensive-document/ demonstrating all block types including horizontalRule, break, definitionList, measurement, signature, svg, barcode, and figure
• Spec updates: Updated spec/core/03-content-blocks.md with tableCell block-only requirement and fixed all table examples
• Strategic insights: Added summary of key strategic decisions to docs/design-decisions.md
• Archive: Moved raw strategy conversation notes to docs/archive/

Test plan

• All schemas compile successfully with ajv-cli
• All example documents validate against their schemas
• horizontalRule and break blocks validate correctly
• Cross-schema references resolve properly