docs: add implementation plan for SSC-13 Assignment Brainstorming & Outline Generator

Comprehensive plan covering:
- Database schema for Assignment, Brainstorm, Outline, ArgumentMap models
- Backend services for brainstorming, outline generation, argument mapping
- Citation and editing assistance services
- 22 atomic commits implementation strategy
- API endpoints documentation
- Frontend component structure
- Academic integrity requirements
- Success criteria and testing strategy

Author: Sakeeb91