docs(tui): add TUI Agent Monitor user guide documentation

[JeremyDev87]

Summary

• Add comprehensive TUI Agent Monitor documentation covering user guide, architecture, and troubleshooting (closes [Phase 4] TUI User Guide Documentation #351)
• User guide includes --tui flag usage, environment variables, supported terminals, Nerd Font installation, and UI components overview
• Architecture doc covers component structure, 7-event system with Mermaid diagrams, event flow, data collection, and bootstrap flow
• Troubleshooting guide addresses icon rendering, color issues, stdout conflicts, TUI not rendering, terminal size, and graceful shutdown
• Format eventbus-ui integration test with prettier

Test plan

• Verify all three documentation files render correctly on GitHub (docs/tui-guide.md, docs/tui-architecture.md, docs/tui-troubleshooting.md)
• Verify Mermaid diagrams render in GitHub markdown preview
• Verify cross-reference links between documentation files resolve correctly
• Confirm no broken relative links in Related Documentation sections
• Verify implementation plan document is included for reference (docs/plans/2026-02-14-tui-user-guide-documentation.md)