docs: add OpenUI token cost comparison article

[nicovaleops]

Closes #4.

Summary

• Adds a developer-facing analytical article: The Token Cost of Beautiful AI: OpenUI Lang vs. AI SDK vs. JSON.
• Uses the program's existing Articles/ folder convention.
• Covers benchmark token counts, estimated output latency, cost projection formula, where compact DSLs help, where AI SDK orchestration still wins, and when plain JSON is the safer tradeoff.
• Keeps the tone practical and technical rather than promotional.

Verification

• Self-reviewed against the issue brief and acceptance points.
• Markdown-only content change; no code/runtime changes.