docs(architecture): add real-time observability architecture for dashboard

- Create dashboard-realtime.md with complete architecture spec
- Define high-level events only (agent, command, session, story)
- Document Claude Code Hooks integration (Decision #1)
- Implement configurable retention settings (Decision #3)
- Add simplified event emitter, store, and UI components
- Include phased implementation plan (~13.5h total)

Decisions made:
- Source: Claude Code Hooks (native integration)
- Detail level: High-level only (8 event types)
- Retention: Configurable (session/hours/persistent)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: oalanicolas