docs: add comprehensive POC implementation plan

- Create detailed 13-phase plan for first POC version
- Include manual workflow orchestration via Claude Code
- Cover matplotlib and seaborn implementations
- Define self-review loop, multi-version testing, and preview optimization
- Specify GCS upload, PostgreSQL metadata, API, and minimal frontend
- Provide code examples, scripts, and troubleshooting guide
- Estimate 4-5 hours total implementation time

This plan validates the end-to-end workflow before automating with GitHub Actions.

Author: claude