Cleanup: Rebuild AGENTS.md and remove legacy docs

- Rebuilt AGENTS.md from scratch (398 lines, AI agent-focused)
  - Added practical code patterns and workflows
  - Added "Before Committing" checklist section
  - Removed outdated historical content
  - Updated architecture diagram and metrics

- Deleted outdated documentation files
  - docs/API.md (1,083 lines)
  - roadmap.md (952 lines)
  - run-demo.sh (83 lines)

- Updated references in README.md

Net reduction: ~2,120 lines
All tests passing ✅

Author: sreya