docs: add messaging & notification center design specification

Comprehensive design document evaluating Slack/Teams/Chatter-like features
for ObjectStack Mobile, including:
- Competitive feature analysis
- Tiered feature scope (Notification Center, Activity Feed, Messaging)
- Architecture and data model design
- Hook and component specifications
- 11-16 week phased development timeline
- @objectstack/client SDK improvement requirements

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

Author: Copilot