feat(FN-4296): complete Step 1 — add task moved agent link sync module

Fusion-Task-Id: FN-4296
Fusion-Task-Lineage: 9d061861-547a-4905-8fdd-4b66dd7bff97

packages/engine/src/task-agent-sync.ts

@@ -0,0 +1,45 @@
+import type { AgentStore, TaskStore } from "@fusion/core";
+
+type LoggerLike = { log: (msg: string) => void; warn: (msg: string) => void };
+
+export interface AttachAgentLinkSyncOptions {
+  store: TaskStore;
+  agentStore: AgentStore;
+  hasActiveAgentExecution?: (agentId: string) => boolean;
+  logger?: LoggerLike;
+}
+
+const CLEAR_COLUMNS = new Set(["done", "archived", "todo", "triage"]);
+
+export function attachAgentLinkSync(opts: AttachAgentLinkSyncOptions): () => void {
+  const logger: LoggerLike = opts.logger ?? console;
+
+  const handler = async ({ task, from, to }: { task: { id: string }; from: string; to: string }) => {
+    if (!CLEAR_COLUMNS.has(to)) {
+      return;
+    }
+
+    try {
+      const agents = await opts.agentStore.listAgents({ includeEphemeral: false });
+      const linkedAgents = agents.filter((agent) => agent.taskId === task.id);
+
+      for (const agent of linkedAgents) {
+        if ((to === "todo" || to === "triage") && opts.hasActiveAgentExecution?.(agent.id) === true) {
+          continue;
+        }
+
+        await opts.agentStore.syncExecutionTaskLink(agent.id, undefined);
+        logger.log(`taskAgentLinkSync: cleared agent ${agent.id} taskId from ${task.id} after move ${from} → ${to}`);
+      }
+    } catch (error) {
+      logger.warn(
+        `taskAgentLinkSync: failed to sync agents for task ${task.id} after move ${from} → ${to}: ${error instanceof Error ? error.message : String(error)}`,
+      );
+    }
+  };
+
+  opts.store.on("task:moved", handler);
+  return () => {
+    opts.store.off("task:moved", handler);
+  };
+}