[taskboard] add heartbeat task pickup

app/api/heartbeat/[agent]/route.ts

@@ -0,0 +1,19 @@
+import { NextResponse } from "next/server";
+
+import { processAgentHeartbeat } from "@/lib/heartbeat";
+
+export async function GET(
+  _request: Request,
+  { params }: { params: Promise<{ agent: string }> },
+) {
+  const { agent } = await params;
+
+  try {
+    const result = await processAgentHeartbeat(agent);
+    return NextResponse.json(result);
+  } catch (error) {
+    const message = error instanceof Error ? error.message : String(error);
+    const status = message === "agent_not_found" ? 404 : 500;
+    return NextResponse.json({ error: message }, { status });
+  }
+}