fix(runtime): cancel de background task immédiat et fiable depuis le workstate (#199, QA verte)

Transforme l'annulation d'une background task en contrat immédiat, observable et non-best-effort:

- domain/background_task: états Cancelling/Cancelled/CancelFailed et transitions idempotentes; un kill PTY qui échoue est surfacé en CancelFailed plutôt qu'en succès silencieux.

- infrastructure/runner + store: cancel effectif de la task ciblée, reconcile marque Cancelling même sans live handle, et départage CancelFailed des tâches pending au boot.

- application/background + workstate + orchestrator/wake: chemin de cancel initié depuis le workstate, projection synchronisée de l'état (pas d'attente d'un cycle ambigu).

- frontend: projection workstate de l'état cancelled/cancelling/cancelFailed (cancellingRow/cancelFailedRow/cancelButtons), normalization et services.

Tests QA verts: background_task_cancel, background_task_runner cancel_wins_even_if_output_is_still_available, background_task_store reconcile_marks_cancelling_without_live_handle_cancel_failed_and_pending, workstate.test.tsx.

frontend/src/features/workstate/workstate.test.tsx

@@ -224,6 +224,21 @@ describe("ProjectWorkStatePanel", () => {
               createdAtMs: 16,
               updatedAtMs: 17,
             },
+            {
+              taskId: "task-cancelling-1",
+              kind: "command",
+              state: "cancelling",
+              createdAtMs: 18,
+              updatedAtMs: 19,
+            },
+            {
+              taskId: "task-cancel-failed-1",
+              kind: "command",
+              state: "cancelFailed",
+              summary: "kill denied",
+              createdAtMs: 20,
+              updatedAtMs: 21,
+            },
           ],
         },
       ],
@@ -237,6 +252,8 @@ describe("ProjectWorkStatePanel", () => {
       ["task-queued-1", "pending"],
       ["task-failed-1", "failed"],
       ["task-expired-1", "cancelled"],
+      ["task-cancelling-1", "cancelling"],
+      ["task-cancel-failed-1", "cancelFailed"],
     ]);
     // Optional fields omitted by the backend normalize to null (not undefined/NaN).
     expect(tasks[0]?.exitCode).toBeNull();
@@ -249,25 +266,44 @@ describe("ProjectWorkStatePanel", () => {
     expect(tasks[1]?.targetAgentId).toBe("agent-bg");
     expect(tasks[1]?.conversationId).toBe("conversation-rdv");
     // updatedAtMs (backend-emitted) is carried through for chronological ordering.
-    expect(tasks.map((t) => t.updatedAtMs)).toEqual([11, 13, 15, 17]);
+    expect(tasks.map((t) => t.updatedAtMs)).toEqual([11, 13, 15, 17, 19, 21]);
 
     renderPanel(workState);
 
     const list = await screen.findByLabelText("Runner background tasks");
     const cancelButtons = within(list).getAllByRole("button", { name: "Cancel" });
     const retryButtons = within(list).getAllByRole("button", { name: "Retry" });
-    // waiting + queued are cancellable; failed + expired are retryable.
-    expect(cancelButtons.filter((b) => !(b as HTMLButtonElement).disabled)).toHaveLength(2);
-    expect(retryButtons.filter((b) => !(b as HTMLButtonElement).disabled)).toHaveLength(2);
+    // waiting + queued + cancelling are cancellable; failed + expired + cancelFailed are retryable.
+    expect(cancelButtons.filter((b) => !(b as HTMLButtonElement).disabled)).toHaveLength(3);
+    expect(retryButtons.filter((b) => !(b as HTMLButtonElement).disabled)).toHaveLength(3);
+    expect(list.textContent).toContain("Cancelling");
+    expect(list.textContent).toContain("Cancel failed");
+    const rows = within(list).getAllByRole("listitem");
+    const rowTexts = rows.map((row) => row.textContent ?? "");
+    expect(rowTexts).toHaveLength(6);
+    const cancelFailedRow = rows[0] as HTMLElement;
+    const cancellingRow = rows[1] as HTMLElement;
+    expect(within(cancellingRow).getByText("Cancelling")).toBeTruthy();
+    expect(
+      (within(cancellingRow).getByRole("button", {
+        name: "Cancel",
+      }) as HTMLButtonElement).disabled,
+    ).toBe(false);
+    expect(within(cancelFailedRow).getByText("Cancel failed")).toBeTruthy();
+    expect(
+      (within(cancelFailedRow).getByRole("button", {
+        name: "Retry",
+      }) as HTMLButtonElement).disabled,
+    ).toBe(false);
 
     // Rows are ordered most-recently-updated first (updatedAtMs desc), taskId as
     // the deterministic tiebreak — locks the fix for the dead finishedAtMs sort.
-    const text = list.textContent ?? "";
-    const order = ["task-exp", "task-fai", "task-que", "task-wai"].map((id) =>
-      text.indexOf(id),
-    );
-    expect(order).toEqual([...order].sort((a, b) => a - b));
-    expect(order.every((i) => i >= 0)).toBe(true);
+    expect(rowTexts[0]).toContain("Cancel failed");
+    expect(rowTexts[1]).toContain("Cancelling");
+    expect(rowTexts[2]).toContain("Cancelled");
+    expect(rowTexts[3]).toContain("Failed");
+    expect(rowTexts[4]).toContain("Pending");
+    expect(rowTexts[5]).toContain("Pending");
   });
 
   it("shows a terminal task summary and hides it when absent or non-terminal (#5)", async () => {