docs: add plain text UI failures article

[nicovaleops]

Closes #5

Summary

• Adds a practical article comparing five AI-output cases that work poorly as plain text and better as structured OpenUI surfaces.
• Covers comparisons, debugging traces, approvals, multi-step workflows, and progress/status output.
• Frames OpenUI as a concrete way to turn model output into scannable, actionable UI without requiring the reader to learn OpenUI internals first.

Validation

• npx --yes markdownlint-cli "Articles/plain-text-ui-failures-openui-fixes.md"
• git diff --check