Migrate Nullable References skill: what it does and how to install it

What it does. Assesses C# language and project readiness, selects a project-wide or file-by-file rollout, enables nullable reference types and works through CS86xx warnings. It guides annotation of APIs, fields and generic returns, cleans up suppressions, runs clean build checkpoints, and can use a readiness scanner.

When to use it. For adopting nullable reference types, updating dependencies with nullable annotations, or auditing suppressions in an existing C# codebase. It does not apply to C# 7.3 or earlier projects, projects already migrated with zero warnings, or warning suppression without fixes.