chore: add .editorconfig and pinned CSharpier tool manifest

Mirrors CSharpier conventions (4-space indent, Allman braces,
System-first usings, 100-col wraps) so IDE auto-format and
'dotnet format' agree with the canonical formatter.
Usage: dotnet tool restore && dotnet csharpier format .

.config/dotnet-tools.json

@@ -0,0 +1,13 @@
+{
+  "version": 1,
+  "isRoot": true,
+  "tools": {
+    "csharpier": {
+      "version": "1.3.0",
+      "commands": [
+        "csharpier"
+      ],
+      "rollForward": false
+    }
+  }
+}