Add dotnet-backend plugin for .NET 9 backend development

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Summary

• Adds a new dotnet-backend plugin providing decision-tree guidance for .NET 9 backend development
• Covers architecture selection (Clean Architecture, Vertical Slice, Modular Monolith), CQRS with MediatR, DDD patterns, EF Core 9 migrations, testing strategies, error handling, observability, and SignalR real-time patterns
• Includes 1 SKILL.md, 3 stack templates, and 6 reference guides (10 files total)
• Registers the plugin in marketplace.json

Plugin Structure

plugins/dotnet-backend/
  skills/dotnet-backend/
    SKILL.md                          # Main skill with interactive architecture decision flow
    stacks/
      clean-architecture.md           # 4-project Clean Architecture scaffolding
      vertical-slice.md               # Feature-per-file architecture
      modular-monolith.md             # Module-based architecture with per-module DbContext
    references/
      ddd-patterns.md                 # Domain-Driven Design tactical patterns
      testing-strategies.md           # Architecture-specific testing (xUnit, Testcontainers, Respawn)
      ef-migrations.md                # EF Core 9 migration best practices
      error-handling.md               # IExceptionHandler + ProblemDetails
      observability.md                # OpenTelemetry + Serilog structured logging
      signalr-patterns.md             # Real-time communication patterns

Key Design Decisions

• All code targets .NET 9 with current NuGet packages
• MediatR v12+ patterns (AddOpenBehavior, INotification)
• NSubstitute for mocking (consistent across all examples)
• SQL Server as default database with Testcontainers for integration tests
• 422 Unprocessable Entity for validation errors (not 400)
• IExceptionHandler (.NET 9 native) instead of middleware for error handling
• Cross-file consistency verified across all 10 files

Test plan

• Verify SKILL.md frontmatter renders correctly in Copilot
• Test architecture decision flow with sample prompts (e.g., "Create a .NET 9 API with Clean Architecture")
• Verify stack templates produce compilable project structures
• Confirm marketplace.json schema is valid
• Test with Copilot Chat to ensure skill activates on .NET backend prompts

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