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/index.html

@@ -30,7 +30,9 @@
         <h3>Create Task</h3>
         <form id="task-form">
           <input id="title" name="title" placeholder="Task title" required />
-          <input id="assignee" name="assignee" placeholder="Assignee agent" />
+          <select id="assignee" name="assignee">
+            <option value="">Select agent...</option>
+          </select>
           <select id="priority" name="priority">
             <option>Low</option>
             <option selected>Medium</option>