chore: add .prettierrc for consistent code formatting

The post-edit-format hook runs Prettier on JS/TS files after edits,
but without a project-level config it applied default settings (double
quotes, etc.) that conflicted with the existing code style. Adding
.prettierrc ensures the hook respects the project conventions.

Settings derived from existing codebase analysis:
- singleQuote: true
- trailingComma: none
- arrowParens: avoid
- printWidth: 200

.prettierrc

@@ -0,0 +1,8 @@
+{
+  "singleQuote": true,
+  "trailingComma": "none",
+  "semi": true,
+  "tabWidth": 2,
+  "printWidth": 200,
+  "arrowParens": "avoid"
+}