Complete comprehensive marketplace publishing documentation

- Added full 9-section design document (595 lines)
- Architecture overview with package taxonomy and namespace scoping
- Detailed 6-step developer publishing flow
- Customer installation flow with 4 channels (CLI/SDK/API/Studio)
- Lifecycle management with state machine
- 3 publishing strategies for multi-plugin apps (HotCRM reference)
- Security model with 5-level publisher verification
- 6 pricing models with license validation
- 5-phase development roadmap (Q1-Q4 2026)

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot