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.

.prettierrc.json

@@ -0,0 +1,7 @@
+{
+    "printWidth": 120,
+    "tabWidth": 4,
+    "proseWrap": "always",
+    "singleQuote": false,
+    "endOfLine": "lf"
+}