feat: handle StopFailure hook event for API error turns (#224)

Parses [StopFailure Hook] from stderr, emits claude:stop-failure, and
transitions the character to error state with a toast notification.

src/lib/stores/toasts.test.ts

@@ -187,6 +187,33 @@ describe("toastStore", () => {
     });
   });
 
+  describe("addError", () => {
+    it("adds an error toast with the warning icon", () => {
+      toastStore.addError("Something went wrong");
+      const toasts = get(toastStore);
+      expect(toasts).toHaveLength(1);
+      const toast = toasts[0];
+      expect(toast.kind).toBe("info");
+      if (toast.kind === "info") {
+        expect(toast.message).toBe("Something went wrong");
+        expect(toast.icon).toBe("⚠️");
+        expect(typeof toast.id).toBe("string");
+        expect(toast.id.length).toBeGreaterThan(0);
+      }
+    });
+
+    it("auto-dismisses after 6000ms", () => {
+      toastStore.addError("Rate limit reached");
+      expect(get(toastStore)).toHaveLength(1);
+
+      vi.advanceTimersByTime(5999);
+      expect(get(toastStore)).toHaveLength(1);
+
+      vi.advanceTimersByTime(1);
+      expect(get(toastStore)).toHaveLength(0);
+    });
+  });
+
   describe("addUpdate", () => {
     it("adds a persistent update toast with the correct fields", () => {
       toastStore.addUpdate("2.0.0", "1.9.0", "https://example.com/release");