chore: housekeeping — linter & formatter setup

Add .prettierrc.json, .markdownlint.json, .yamllint.yaml, .gitattributes
Run CSharpier, Prettier and markdownlint across the entire codebase.
No logic changes — formatting, using order and line endings only.

dotnet-tools.json

@@ -0,0 +1,11 @@
+{
+    "version": 1,
+    "isRoot": true,
+    "tools": {
+        "csharpier": {
+            "version": "1.2.6",
+            "commands": ["csharpier"],
+            "rollForward": false
+        }
+    }
+}