chore: add coding guidelines rule

.claude/rules/coding-guidelines.md

@@ -0,0 +1,67 @@
+# Coding Guidelines
+
+These guidelines apply regardless of programming language.
+
+## Early Returns
+
+Prefer returning early to reduce nesting. Handle edge cases and error conditions first, then proceed with the main logic.
+
+**Good:**
+
+```javascript
+function processItems(items) {
+  if (!items || items.length === 0) {
+    return [];
+  }
+
+  // main logic here
+  return items.map(transform);
+}
+```
+
+**Avoid:**
+
+```javascript
+function processItems(items) {
+  if (items && items.length > 0) {
+    // main logic here
+    return items.map(transform);
+  } else {
+    return [];
+  }
+}
+```
+
+## Flat Code
+
+Keep code as flat as possible. Avoid deep nesting by using early returns, guard clauses, or extracting logic into separate functions.
+
+**Good:**
+
+```javascript
+async function fetchData(url) {
+  if (!url) return null;
+
+  const response = await fetch(url);
+  if (!response.ok) return null;
+
+  return response.json();
+}
+```
+
+**Avoid:**
+
+```javascript
+async function fetchData(url) {
+  if (url) {
+    const response = await fetch(url);
+    if (response.ok) {
+      return response.json();
+    } else {
+      return null;
+    }
+  } else {
+    return null;
+  }
+}
+```