feat(agent): add task cancellation with stop button

Users can now interrupt running agents via a "Stop" button on the live
card. The daemon polls task status every 5 seconds and kills the agent
process when cancellation is detected.

Changes:
- New CancelAgentTask SQL query and CancelTask service method
- POST /api/issues/{id}/tasks/{taskId}/cancel endpoint
- Daemon polls GetTaskStatus during execution, cancels context on match
- Frontend: Stop button on AgentLiveCard, task:cancelled WS event

apps/web/shared/api/client.ts

@@ -352,6 +352,12 @@ export class ApiClient {
     return this.fetch(`/api/issues/${issueId}/task-runs`);
   }
 
+  async cancelTask(issueId: string, taskId: string): Promise<AgentTask> {
+    return this.fetch(`/api/issues/${issueId}/tasks/${taskId}/cancel`, {
+      method: "POST",
+    });
+  }
+
   async getDaemonPairingSession(token: string): Promise<DaemonPairingSession> {
     return this.fetch(`/api/daemon/pairing-sessions/${token}`);
   }