docs: Add detailed design document for Context refactoring

Create comprehensive design doc covering:
- Problem analysis (SOLID violations, tight coupling)
- Proposed solution (Factory pattern + DI)
- Architecture diagrams
- Phase-by-phase implementation plan
- Risk mitigation strategies
- Future work alignment with jDisco migration

Co-authored-by: bedaHovorka <5263405+bedaHovorka@users.noreply.github.com>

Author: Copilot