migrate-nullable-references

# Nullable Reference Migration Enable C# nullable reference types (NRTs) in an existing codebase and systematically resolve all warnings. The outcome is a project (or solution) with `<Nullable>enable</Nullable>`, zero nullable warnings, and accurately annotated public API surfaces — giving both the compiler and consumers reliable nullability information. ## When to Use - Enabling nullable reference types in an existing C# project or solution - Systematically resolving CS86xx nullable warnings after enabling the feature - Annotating a library's public API surface so consumers get accurate nullability information - Upgrading a dependency that has added nullable annotations and new warnings appear - Analyzing suppressions in a code base that has already enabled NRTs to determine whether they can be removed ## When Not to Use - The project already has `<Nullable>enable</Nullable>` and zero warnings — the migration is done unless the user wants to re-examine suppressions with a view to removing unnecessary ones (see Step 6) - The user only wants to suppress warnings without fixing them (recommend against this) - The code targets C# 7.3 or earlier, which does not support nullable reference types ## Inputs | Input | Required | Description | |-------|----------|-------------| | Project or solution path | Yes | The `.csproj`, `.sln`, or build entry point to migrate | | Migration scope | No | `project-wide` (default) or `file-by-file` — controls the rollout strategy | | Build co...