docs: comprehensive documentation overhaul with visual enhancements

[simonCatBot]

Summary

Complete documentation overhaul with user-friendly formatting, visual diagrams, and comprehensive examples.

Changes Made

• README.md (+82 lines): Added emoji headers, setup guides with collapsible sections, architecture diagrams, and enhanced troubleshooting
• CONTRIBUTING.md (+277 lines): Step-by-step developer setup, comprehensive troubleshooting guide, testing procedures, and PR templates
• ARCHITECTURE.md (+519 lines): Data flow diagrams, API route documentation, security model with trust boundaries, database schema, and design guardrails
• AGENTS.md (+98 lines): AI agent instructions with scope definition, coding guidelines, and security reminders
• ui-guide.md (+263 lines): Visual component diagrams, step-by-step usage instructions, keyboard shortcuts, and troubleshooting by symptom
• permissions-sandboxing.md (+252 lines): Decision flowcharts, layered security model, workspace access comparison tables, and debugging checklists
• pi-chat-streaming.md (+474 lines): Detailed event flow with code examples, reconnection strategy, pagination documentation, and event types reference

Key Improvements

• 📊 Total documentation increased from 901 to 2,866 lines (+218%)
• 📑 All documents now include tables of contents
• 🔽 Collapsible sections for complex topics
• 🎨 Visual ASCII diagrams for architecture and data flow
• 🔗 Cross-references between documents
• 💻 Code examples with syntax highlighting
• ✅ Troubleshooting checklists

Testing

• Documentation renders correctly in Markdown viewers
• All links verified
• Code examples validated

AI-assisted

• AI-assisted (Documentation restructuring and content expansion using Claude)

Note: This is a documentation-only change. No code functionality was modified.