Complete the game agent runtime foundation

[EricSun0218]

Summary

• stabilize the streamed model/tool agent kernel and public API boundaries
• add game-aware sessions, actors, durable actions, resources, providers, authentication, media, and engine/server placement
• add bounded release packaging for 21 NuGet packages and versioned engine/server assets

Verification

• 759 Release tests passed with zero failures or skips
• locked restore and Release build passed with zero warnings
• real DeepSeek quick and tool/action runs passed
• Godot 4.7.1 and Unity 6 real-editor smoke tests passed on Windows
• clean consumer restored and built all 21 NuGet packages
• portable server archive startup smoke passed
• public-tree, secret/reference, Markdown-link, format, and vulnerability gates passed