chore: Add agentic coding tooling

.crosslink/rules/javascript.md

@@ -0,0 +1,36 @@
+### JavaScript Best Practices
+
+#### Code Style
+- Use `const` by default, `let` when needed, never `var`
+- Use arrow functions for callbacks
+- Use template literals over string concatenation
+- Use destructuring for object/array access
+
+#### Error Handling
+```javascript
+// GOOD: Proper async error handling
+async function fetchUser(id) {
+    try {
+        const response = await fetch(`/api/users/${id}`);
+        if (!response.ok) {
+            throw new Error(`HTTP ${response.status}`);
+        }
+        return await response.json();
+    } catch (error) {
+        console.error('Failed to fetch user:', error);
+        throw error;  // Re-throw or handle appropriately
+    }
+}
+
+// BAD: Ignoring errors
+async function fetchUser(id) {
+    const response = await fetch(`/api/users/${id}`);
+    return response.json();  // No error handling
+}
+```
+
+#### Security
+- Never use `eval()` or `innerHTML` with user input
+- Validate all input on both client and server
+- Use `textContent` instead of `innerHTML` when possible
+- Sanitize URLs before navigation or fetch