Generalize project positioning

[EricSun0218]

Summary

• replace the comparison-based tagline with a standalone OpenGameAgent category statement
• describe agent-driven game characters through observable behavior rather than external product names
• keep English and Simplified Chinese positioning aligned

Verification

• both README files render successfully through the GitHub Markdown API
• logo picture, heading, badges, and theme assets remain intact
• old Pi-for-games and Codex/ChatGPT comparison copy is absent from both README files
• git diff --check passes