migrate-dotnet8-to-dotnet9

# .NET 8 → .NET 9 Migration Migrate a .NET 8 project or solution to .NET 9, systematically resolving all breaking changes. The outcome is a project targeting `net9.0` that builds cleanly, passes tests, and accounts for every behavioral, source-incompatible, and binary-incompatible change introduced in the .NET 9 release. ## When to Use - Upgrading `TargetFramework` from `net8.0` to `net9.0` - Resolving build errors or new warnings after updating the .NET 9 SDK - Adapting to behavioral changes in .NET 9 runtime, ASP.NET Core 9, or EF Core 9 - Replacing `BinaryFormatter` usage (now always throws at runtime) - Updating CI/CD pipelines, Dockerfiles, or deployment scripts for .NET 9 ## When Not to Use - The project already targets `net9.0` and builds cleanly — migration is done. If the goal is to reach `net10.0`, use the `migrate-dotnet9-to-dotnet10` skill as the next step. - Upgrading from .NET 7 or earlier — address the prior version breaking changes first - Migrating from .NET Framework — that is a separate, larger effort - Greenfield projects that start on .NET 9 (no migration needed) ## Inputs | Input | Required | Description | |-------|----------|-------------| | Project or solution path | Yes | The `.csproj`, `.sln`, or `.slnx` entry point to migrate | | Build command | No | How to build (e.g., `dotnet build`, a repo build script). Auto-detect if not provided | | Test command | No | How to run tests (e.g., `dotnet test`). Auto-detect if not provided | | Project type ...