build(acl): pin Stryker.NET as a local dotnet tool (refs #47)

Add a tool manifest pinning dotnet-stryker 4.15.0 so `make mutation` runs
the same mutation tester locally and in CI from a fresh clone (`dotnet tool
restore`), with no global install. Ignore the generated StrykerOutput/ report
directory.

Refs #47.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.config/dotnet-tools.json

@@ -0,0 +1,13 @@
+{
+  "version": 1,
+  "isRoot": true,
+  "tools": {
+    "dotnet-stryker": {
+      "version": "4.15.0",
+      "commands": [
+        "dotnet-stryker"
+      ],
+      "rollForward": false
+    }
+  }
+}

.gitignore

@@ -15,6 +15,9 @@ coverage*.json
 coverage*.xml
 *.coverage
 
+# Stryker.NET mutation-testing reports (regenerated by `make mutation`)
+StrykerOutput/
+
 # Rider / VS / VS Code
 .idea/
 .vs/