Freeze the extensible game agent core

[EricSun0218]

Summary

• freeze the compact game-agent kernel and keep optional capabilities in extensions
• add official extensions, model/auth management, external tool connectivity, durable workflows, and developer-hosted credentials
• harden concurrency, persistence, network, streaming, engine, and release boundaries
• update English and Chinese documentation and package/release automation

Validation

• locked dependency restore
• formatting verification
• Release build: 0 warnings, 0 errors
• 359 tests passed
• Godot 4.7.1 .NET editor smoke passed
• Unity 6000.5.6f1 editor smoke passed
• public-tree/privacy scan passed
• dependency vulnerability scan passed