chore: prepare Unity and Godot store packages

[EricSun0218]

Summary

• prepare Unity UPM and Godot 4.7 .NET packages for store review
• add deterministic no-key samples, complete license/privacy/dependency disclosures, and valid Godot plugin metadata
• add canonical listing copy, three 16:9 marketing assets, reproducible rendering, and CI validation

Verification

• dotnet restore OpenGameAgent.sln --locked-mode
• dotnet build OpenGameAgent.sln -c Release --no-restore (0 warnings, 0 errors)
• dotnet test OpenGameAgent.sln -c Release --no-build --no-restore (994 passed)
• dotnet format OpenGameAgent.sln --verify-no-changes --no-restore
• Unity 6000.5.6f1 real-editor smoke
• Godot 4.7.1 .NET real-editor/runtime smoke
• release scripts, public-tree, package, and store-listing gates