Replace ASCII diagrams with Mermaid in ARCHITECTURE.md

[lucastononro]

Summary

• Converted system overview diagram to Mermaid graph LR flowchart
• Converted database schema to Mermaid erDiagram with all tables and relationships
• Converted sandbox execution flow to Mermaid sequenceDiagram
• Converted session state machine to Mermaid stateDiagram-v2
• Left file/directory trees as plain text (better suited for that format)

Test plan

• Verify all four Mermaid diagrams render correctly on GitHub

🤖 Generated with Claude Code