[Phase 2] Implement PowerCSharp.Sitecore core context logic

[marioarce]

Task

Implement the core thread-safe context logic using MarshalByRefObject + ILogicalThreadAffinative pattern.

Deliverables

• Create src/PowerCSharp.Sitecore/PowerCSharp.Sitecore.csproj
• Target frameworks: net48, net6.0, net8.0
• Implement SitecoreThreadSafeContext:
  • Implements ISitecoreThreadSafeContext
  • Implements ILogicalThreadAffinative
  • Marked as [Serializable]
  • Stores context in CallContext (net48) or AsyncLocal (net6/8)
• Implement SitecoreContextService:
  • Extends MarshalByRefObject
  • Static Context property for access
  • Uses CallContext.GetData/SetData for context propagation
• Implement ReflectionSitecoreContextProvider:
  • Uses reflection to access Sitecore.Context at runtime
  • Caches PropertyInfo for performance
  • Gracefully handles missing Sitecore assembly
• Add Database property only (v1.0.0 scope)
• Add README.md
• Add package to PowerCSharp.sln
• Configure CI/CD

Acceptance Criteria

• Context propagates across thread boundaries
• Reflection-based Sitecore access works at runtime
• Graceful failure when Sitecore unavailable
• No compile-time Sitecore.Kernel dependency
• Unit tests pass for all target frameworks