Define dataset schema (ccgram-data-schema-plan)

## Summary
Define the canonical schema for storing conventional commit data in ccgram.

## Success Criteria
- [ ] JSON schema defined for commit records
- [ ] Fields documented: type, scope, subject, body, footer, metadata
- [ ] Schema versioning strategy documented
- [ ] Example records provided for each commit type
- [ ] Validation script can check conformance

## Context
This schema will be the foundation for all data processing pipelines. It should be flexible enough to capture the full conventional commit spec while being strict enough to ensure consistency.

## References
- [Conventional Commits spec](https://www.conventionalcommits.org/)