Add persistent task plan extension

[EricSun0218]

Summary

• add a generic persistent ordered TaskPlanExtension
• require host-validated evidence for step advancement
• enforce revision CAS, once-per-input advancement, pending-work routing, and bounded terminal retention
• add restart, concurrency, evidence, replanning, isolation, event, and retention tests

Validation

• dotnet test OpenGameAgent.sln -c Release --no-restore --nologo (836/836)
• dotnet restore OpenGameAgent.sln --locked-mode --nologo
• dotnet format OpenGameAgent.sln --verify-no-changes --no-restore
• dotnet pack src/OpenGameAgent.Extensions/OpenGameAgent.Extensions.csproj -c Release --no-restore