Stamp interrupted stop reason on send_input interrupt

send_input with interrupt:true already unblocked wait_agents as
interrupted, but only the overlay flipped. Stamp session.stopReason
after the follow-up queues so that wait JSON includes
stop_reason interrupted, matching interrupt_agent.

Author: TheGreatAxios

src/subagent/agent-fleet.test.ts

@@ -1103,8 +1103,13 @@ describe("interrupt_agent unblocks wait_agents", () => {
     await callTool(sendInput, { target: id, message: "stop that", interrupt: true });
     const waited = await waiting;
     expect(waited.timed_out).toBe(false);
-    const results = waited.results as { status: string }[];
-    expect(results[0]!.status).toBe("interrupted");
+    const results = waited.results as {
+      agent_id: string;
+      status: string;
+      stop_reason?: string;
+    }[];
+    expect(results).toEqual([{ agent_id: id, status: "interrupted", stop_reason: "interrupted" }]);
+    expect(deps.sessions.get(id)?.stopReason).toBe("interrupted");
     followupGate.resolve("later");
   });
 

src/subagent/session-store.test.ts

@@ -308,6 +308,19 @@ describe("terminal stop reasons", () => {
     expect(store.interruptOne(session.id).ok).toBe(true);
     expect(store.get(session.id)?.stopReason).toBe("interrupted");
   });
+
+  test("sendInputOne interrupt records stopReason interrupted", () => {
+    const store = createSubAgentSessionStore();
+    const session = store.start({ description: "d", agentId: "a", brief: "b", retained: true });
+    store.markRunning(session.id);
+    store.registerInterrupt(session.id, () => {});
+    store.registerFollowup(session.id, () => new Promise(() => {}));
+    expect(store.sendInputOne(session.id, "stop that", { interrupt: true })).toEqual({
+      ok: true,
+      status: "interrupted",
+    });
+    expect(store.get(session.id)?.stopReason).toBe("interrupted");
+  });
 });
 
 describe("CL-6943 reusable worker sessions", () => {

src/subagent/session-store.ts

@@ -1303,6 +1303,12 @@ export function createSubAgentSessionStore(
         queueFollowupTurn(id, message, "interrupted", {
           ...(opts.onFollowupReply !== undefined ? { onReply: opts.onFollowupReply } : {}),
         });
+        // After beginFollowupTurn, which clears leftover stopReason. Stamp
+        // here so an in-flight wait_agents overlay can project interrupted
+        // without flipping lifecycle off the live follow-up.
+        mutate(id, (s) => {
+          s.stopReason = "interrupted";
+        });
         pruneRetained();
         return { ok: true, status: "interrupted" };
       }