feat: add agent dropdown for task assignment

- Replace assignee text input with select dropdown
- Populate dropdown with agents from /api/agents endpoint
- Add styling to match existing form elements
- Improves UX by preventing typos and showing available agents

public/styles.css

@@ -529,3 +529,30 @@ body {
     max-height: none;
   }
 }
+
+/* Agent Dropdown Styling */
+#task-form select {
+  width: 100%;
+  padding: 0.75rem;
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  font-size: 1rem;
+  background: var(--bg);
+  color: var(--fg);
+  cursor: pointer;
+  transition: border-color 0.2s ease, box-shadow 0.2s ease;
+}
+
+#task-form select:hover {
+  border-color: var(--primary);
+}
+
+#task-form select:focus {
+  outline: none;
+  border-color: var(--primary);
+  box-shadow: 0 0 0 3px rgba(52, 152, 219, 0.1);
+}
+
+#task-form select option {
+  padding: 0.5rem;
+}