test(lanes): add test for stale run_id clearing during healing

Add missing test coverage for healRunId's core behavior: that a STALE run_id
(tmux session gone) gets CLEARED to null with status: idle when read via GET.
The existing test only verified the LIVE case (session still running). This test
proves the release-on-gone path, simulating a session death via tmux mock.

server/__tests__/lane-lifecycle.test.js

@@ -1163,4 +1163,38 @@ describe("lane ensure, start mode, lane_id and releasing a finished run", () =>
       tmux.__reset();
     }
   });
+
+  it("clears a stale run_id and sets status to idle when the tmux session is gone", async () => {
+    const tmux = require("../lib/tmux");
+    const lane = await adoptedLane("release-stale-run");
+
+    // Start a run for this lane.
+    const started = await request("POST", `/api/lanes/${lane.id}/start`, { prompt: "test" });
+    assert.equal(started.status, 200, JSON.stringify(started.body));
+    const runId = started.body.lane.run_id;
+    assert.equal(typeof runId, "string");
+    assert.equal(started.body.lane.status, "running");
+
+    // Mock tmux so the session appears to be gone (has-session fails with status 1).
+    tmux.__setExecImpl((args) => {
+      if (args[0] === "has-session" && args[1] === "-t" && args[2] === runId) {
+        const e = new Error("no such session");
+        e.status = 1;
+        throw e;
+      }
+      if (args[0] === "list-sessions") {
+        return "";
+      }
+      return "";
+    });
+
+    try {
+      // Read the lane — it should clear the run_id and set status to idle.
+      const after = (await request("GET", `/api/lanes/${lane.id}`)).body.lane;
+      assert.equal(after.run_id, null, "run_id should be cleared for stale session");
+      assert.equal(after.status, "idle", "status should be idle after run is gone");
+    } finally {
+      tmux.__reset();
+    }
+  });
 });