Add skill for using dotnet format whitespace

plugins/dotnet/skills/dotnet-format/SKILL.md

@@ -0,0 +1,85 @@
+---
+name: dotnet-format
+description: >
+  Format C#/.NET source files using the `dotnet format whitespace` subcommand.
+  USE FOR: quickly fixing whitespace and indentation in one or more files after
+  code generation or editing, batch-formatting changed files before a commit.
+  DO NOT USE FOR: enforcing code-style analyzers (SA/IDE rules), fixing
+  non-whitespace style issues, or formatting entire large repositories at once.
+---
+
+# dotnet format (whitespace)
+
+`dotnet format whitespace` reformats indentation, trailing whitespace, and line endings in C#/VB files without applying analyzer or code-style fixes. When used with `--folder`, it is the fastest `dotnet format` sub-command because in that mode it operates on syntax only and does not need to load the full MSBuild workspace; when run against a project or solution it still loads the workspace. It is also the safest because it never changes semantics.
+
+## When to Use
+
+- Fixing indentation or trailing whitespace in files you just created or edited
+- Batch-formatting a set of changed files before committing
+- Cleaning up generated code that has inconsistent whitespace
+
+## When Not to Use
+
+- You need code-style fixes (naming, `var` vs explicit type, etc.) — use `dotnet format style` instead
+- You need analyzer-driven fixes (e.g., SA1200) — use `dotnet format analyzers` instead
+
+## Workflow
+
+### Step 1: Determine files to format
+
+Identify the file paths that need formatting — typically files that were just created or edited.
+
+### Step 2: Run `dotnet format whitespace`
+
+Use `--folder` mode to format without needing a project/solution file:
+
+```bash
+# Format everything under the current directory
+dotnet format whitespace --folder .
+
+# Format only specific files
+dotnet format whitespace --folder . --include path/to/File1.cs --include path/to/File2.cs
+```
+
+Key flags:
+
+Always use `--folder` — without it, the tool loads the full MSBuild workspace which is much slower and unnecessary for whitespace-only formatting.
+
+| Flag | Purpose |
+|------|---------|
+| `--folder` | Treats the argument as a plain directory — avoids loading the full workspace |
+| `--include <path>` | Restricts formatting to the specified file(s); repeat for multiple files. Optional — omit to format all files in the folder |
+| `--verify-no-changes` | Exits non-zero if any file would change (useful for CI checks) |
+
+Multiple files example:
+
+```bash
+dotnet format whitespace --folder . \
+  --include src/Models/User.cs \
+  --include src/Services/AuthService.cs \
+  --include tests/AuthTests.cs
+```
+
+### Step 3: Verify the result
+
+Review the formatted files to confirm only whitespace changed. If you need to verify programmatically:
+
+```bash
+dotnet format whitespace --folder . --include path/to/File.cs --verify-no-changes
+```
+
+A zero exit code means the file is already correctly formatted.
+
+## Validation
+
+- [ ] `dotnet format whitespace` exits with code 0
+- [ ] Only whitespace/indentation changed — no semantic modifications
+- [ ] Formatted files still compile successfully
+
+## Common Pitfalls
+
+| Pitfall | Solution |
+|---------|----------|
+| Running without `--folder` | Loads the full MSBuild workspace, which is slow and unnecessary for whitespace formatting. Always use `--folder .` |
+| Formatting the entire repo unintentionally | Pass `--include` with specific file paths when you only want to format a subset |
+| `.editorconfig` not found | `dotnet format` inherits `.editorconfig` settings; ensure one exists in a parent directory for consistent results |

tests/dotnet/dotnet-format/eval.yaml

@@ -0,0 +1,37 @@
+scenarios:
+  - name: "Format specific files after editing"
+    prompt: |
+      I just refactored two C# files and the indentation is all messed up:
+      - src/Services/OrderService.cs
+      - src/Models/Order.cs
+
+      I don't have a solution file in the current directory. Can you fix the whitespace formatting?
+    assertions:
+      - type: "output_contains"
+        value: "dotnet format whitespace"
+      - type: "output_contains"
+        value: "--folder"
+      - type: "output_matches"
+        pattern: "--include"
+    rubric:
+      - "Uses `dotnet format whitespace` specifically, not `dotnet format` without a subcommand or `dotnet format style`"
+      - "Uses --folder mode to avoid loading the MSBuild workspace"
+      - "Uses --include to target the specific files rather than formatting the entire directory"
+    timeout: 120
+
+  - name: "Verify whitespace formatting without modifying files"
+    prompt: |
+      I just ran `dotnet format whitespace` on some files in my repo but I want to
+      double-check that `src/Services/PaymentService.cs` is now correctly formatted.
+      Can you verify the formatting without modifying the file? I don't have a .sln
+      file in this directory.
+    assertions:
+      - type: "output_contains"
+        value: "dotnet format whitespace"
+      - type: "output_contains"
+        value: "--verify-no-changes"
+    rubric:
+      - "Uses `dotnet format whitespace` with `--verify-no-changes` to verify without modifying the file"
+      - "Uses --folder mode since there is no solution file available"
+      - "Uses --include to target the specific file rather than checking the entire directory"
+    timeout: 120