chore: remove demo implementation phases documentation (#373)

# Remove Implementation Phases Documentation

This PR removes all implementation phase documentation files that were used as guides during the development of the demo application. The following files have been deleted:

- `START_HERE.md` - Initial overview and branch setup guide
- `PHASE_0_FOUNDATION.md` - SvelteKit app setup and Nx integration
- `PHASE_1_VERTICAL_SLICE.md` - End-to-end flow execution implementation
- `PHASE_2_ARTICLE_FLOW.md` - Article processing flow with state management
- `PHASE_3_DAG_DEBUG.md` - DAG visualization and debug panel implementation
- `PHASE_4_CODE_EXPLANATION.md` - Code panel with syntax highlighting
- `PHASE_5_RESULTS_MODALS.md` - Results card and output modal implementation
- `PHASE_6_POLISH_DEPLOY.md` - Final polish and deployment instructions

These files served as step-by-step implementation guides during development but are no longer needed now that the demo application is complete.

Author: jumski