docs: Add Mermaid diagrams for architecture visualization

[whitmo]

Summary

• Adds docs/DIAGRAMS.md with 12 Mermaid diagrams covering all major system flows
• Diagrams render in GitHub's Markdown viewer for easy navigation
• Complements existing ASCII diagrams in ARCHITECTURE.md with interactive visualizations

Diagrams included

1. System Overview - High-level component architecture
2. Brownian Ratchet - Core philosophy visualization
3. Agent Types and Relationships - How agents interact
4. Worker Lifecycle - State machine from spawn to cleanup
5. Worker Creation Flow - Sequence diagram of spawn process
6. Message Routing - How messages flow between agents
7. Message Status Flow - Status state machine
8. Repository Initialization - Init sequence diagram
9. Health Check Loop - Flowchart of health check logic
10. Daemon Goroutines - Background loop structure
11. File System Layout - Directory tree visualization
12. Shutdown Sequence - Graceful shutdown flow
13. State Thread Safety - Mutex and concurrent access model

Test plan

• Verify diagrams render correctly on GitHub
• Review diagram accuracy against ARCHITECTURE.md

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