Persist error/interrupt markers in history; add Retry on failed turns

Engine appends a display-only notice message on error/interrupted; providers never see it.
New retry frame re-runs a failed turn with no new user message, guarded on the error tail.
GUI renders persisted notices on reload and a Retry button on the trailing error.

surfaces/gui/e2e/error-retry.spec.ts

@@ -0,0 +1,27 @@
+// Model-layer roadmap item 1 (2026-07-22): a turn that dies on a provider error leaves a
+// visible, persistent marker with a Retry affordance. Retry re-runs the failed turn with NO
+// new user bubble; once the turn recovers, the button disappears (the notice is history).
+import { expect } from "@playwright/test";
+import { test } from "./fixtures";
+
+test("provider error shows a retriable notice; Retry re-runs without a new user message", async ({
+  page,
+}) => {
+  await page.goto("/");
+  await page.getByText("Draft the launch note").first().click();
+  const box = page.getByPlaceholder(/Ask the coworker/);
+  await box.fill("please fail the turn");
+  await box.press("Enter");
+
+  await expect(page.getByText("Error: model unreachable").first()).toBeVisible({ timeout: 10_000 });
+  const retry = page.getByTestId("notice-retry");
+  await expect(retry).toBeVisible();
+
+  await retry.click();
+  await expect(page.getByText("Recovered after retry.").first()).toBeVisible({ timeout: 10_000 });
+
+  // No fake user bubble from the retry turn, exactly one real one…
+  await expect(page.locator(".bubble-user")).toHaveCount(1);
+  // …and the button is gone now that the error notice is no longer the transcript tail.
+  await expect(page.getByTestId("notice-retry")).toHaveCount(0);
+});

surfaces/gui/e2e/fixtures.ts

@@ -646,6 +646,12 @@ export async function mockApi(page: import("@playwright/test").Page) {
           });
           return;
         }
+        // A turn that dies on a provider error; the follow-up {type:"retry"} recovers.
+        if (/fail the turn/i.test(msg.text)) {
+          send("error", { error: "model unreachable" });
+          send("turn_done");
+          return;
+        }
         // A deliberately SLOW multi-second stream (~40 ticks × 120ms) so specs can
         // interact mid-turn — the follow/pin scroll contract (FB-004) is untestable
         // against the instant echo below.
@@ -697,6 +703,11 @@ export async function mockApi(page: import("@playwright/test").Page) {
         }
         send("interrupted", {});
         send("turn_done");
+      } else if (msg.type === "retry") {
+        // Like the real engine: re-runs with NO new user message (turn_start input is empty).
+        send("turn_start", { input: "" });
+        send("assistant_message", { text: "Recovered after retry." });
+        send("turn_done");
       }
     });
   });