docs: Add comprehensive TUI module documentation

[inureyes]

Summary

This PR adds comprehensive documentation for the Terminal User Interface (TUI) module that was implemented in issue #68. The documentation covers both the technical architecture (for developers) and user-facing features (for end users).

Documentation Updates

ARCHITECTURE.md

Added extensive TUI architecture section (~500 lines) covering:

• Module structure: Complete file hierarchy and component descriptions
• ViewMode state machine: Summary, Detail, Split, and Diff view modes
• Event handling: Keyboard input processing with mode-specific handlers
• RollingBuffer design: Memory protection mechanism (10MB per node)
• Event loop architecture: Non-blocking polling and lazy rendering
• State management: Change detection, scroll positions, follow mode
• Performance characteristics: Target metrics and optimization strategies
• Terminal safety: RAII guards for panic-safe cleanup
• Output mode auto-detection: TUI activation conditions
• Testing strategy: Unit test coverage and integration testing approach
• Known limitations: Documented TODOs and future enhancements

README.md

Enhanced TUI documentation with user-friendly reference:

• View modes table: Description and access keys for all 4 views
• Comprehensive keyboard shortcuts table: Organized by context (Global, Summary, Detail, Split, Diff)
• TUI activation conditions: When TUI is automatically enabled/disabled
• Feature highlights: Progress detection, memory limits, real-time updates
• Minimum requirements: Terminal size requirements (40x10)

src/ui/tui/app.rs

Fixed missing keyboard shortcut in help overlay:

• Added "1-9: Jump to node N" in detail view help text
• Ensures help overlay matches actual available shortcuts

Related Issues

• Resolves Add TUI module documentation to ARCHITECTURE.md and README #81 (Add TUI module documentation)
• Related to feat: Implement real-time streaming output with interactive multi-node UI #68 (Real-time streaming output with interactive multi-node UI)

Test Plan

• Documentation formatting verified (markdown tables, code blocks)
• Keyboard shortcuts table matches implementation in event.rs
• Help overlay includes all available shortcuts
• ARCHITECTURE.md structure follows existing patterns
• No code changes beyond help text addition
• Pre-commit hooks passed (cargo fmt, clippy)

Checklist

• Documentation follows existing style and conventions
• All keyboard shortcuts documented and verified
• Help overlay updated with complete shortcuts
• ARCHITECTURE.md section added after Interactive Mode
• README.md TUI section enhanced with tables
• Commit message follows conventional commits format
• Issue Add TUI module documentation to ARCHITECTURE.md and README #81 resolved in commit message