Add JIT regression test extraction skill

.github/skills/jit-regression-test/SKILL.md

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+---
+name: jit-regression-test
+description: Extract a standalone JIT regression test case from a given GitHub issue and save it under the JitBlue folder. Use this when asked to create or extract a JIT regression test from an issue.
+---
+
+# JIT Regression Test Extraction
+
+When you need to extract a JIT regression test case from a GitHub issue, follow this process to create a properly structured test under `src/tests/JIT/Regression/JitBlue/`.
+
+## Step 1: Gather Information from the GitHub Issue
+
+From the GitHub issue, extract:
+1. **Issue number** - Used to name the test folder and files (e.g., issue #99391 → `Runtime_99391`)
+2. **Reproduction code** - The C# code that demonstrates the bug. If no code provided, try to compose it yourself.
+3. **Environment variables** - Any DOTNET_* environment variables required to reproduce the bug
+4. **Expected behavior** - What the correct output/behavior should be
+
+## Step 2: Create the Test Directory
+
+Create a new folder under `src/tests/JIT/Regression/JitBlue/`:
+
+```
+src/tests/JIT/Regression/JitBlue/Runtime_<issue_number>/
+```
+
+## Step 3: Create the Test File
+
+Create a `Runtime_<issue_number>.cs` file following these conventions:
+
+Example:
+
+```csharp
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_<issue_number>
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        // Test code that exercises the bug
+        // Use Assert.Equal, Assert.True, etc. for validation
+    }
+}
+```
+
+### Key Conventions
+
+- **License header**: Always include the standard .NET Foundation license header
+- **Class name**: Match the file name exactly (`Runtime_<issue_number>`)
+- **Test method**: Use `[Fact]` attribute and name the method `TestEntryPoint()` or `Test()`
+- **Assertions**: Use Xunit's helpers or just throw plain exceptions.
+
+### Example: Simple Test (from Runtime_99391)
+
+```csharp
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+
+namespace Runtime_99391;
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Numerics;
+using Xunit;
+
+public class Runtime_99391
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        Vector2 result2a = Vector2.Normalize(Value2);
+        Assert.Equal(new Vector2(0, 1), result2a);
+    }
+
+    private static Vector2 Value2
+    {
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        get => new Vector2(0, 2);
+    }
+}
+```
+
+## [Optional] Step 4: Create a .csproj File (Only When Needed)
+
+A custom `.csproj` file is **only required** when:
+- Environment variables are needed to reproduce the bug (such as `DOTNET_JitStressModeNames`)
+- Special compilation settings are required
+
+It should be located next to the test source file with the following name: `Runtime_<issue_number>.csproj`. Example:
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+    <DebugType>None</DebugType>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredCompilation" Value="0" />
+  </ItemGroup>
+</Project>
+```
+
+## Important Notes
+
+- **No .csproj needed for simple tests**: Most tests only need the `.cs` file. The test infrastructure uses default settings that work for most cases.
+- **Look at recent tests**: When in doubt, examine recent tests under `src/tests/JIT/Regression/JitBlue/Runtime_*` for the latest conventions.
+- **Use `[MethodImpl(MethodImplOptions.NoInlining)]`**: When you need to prevent inlining to reproduce a JIT bug.
+- **Minimize the reproduction**: Strip down the test code to the minimal case that reproduces the issue.