feat(background-task): rendu live des tâches de fond — subscriber UI + canal IPC (#58)

Câble un canal attachable au flux de sortie d'une tâche de fond (runner
infrastructure + commande app-tauri + port/adaptateurs frontend) et le
panneau ProjectWorkStatePanel s'y abonne pour un rendu live au lieu d'un
état figé au dernier snapshot.

Validations obtenues avant commit :
- cargo test -p infrastructure --test background_task_runner : vert
- cargo check -p backend -p app-tauri : vert
- npx vitest run src/features/workstate/workstate.test.tsx : vert
- npm run typecheck : vert
- verdict QA #58 : vert

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/ports/index.ts

@@ -388,6 +388,18 @@ export interface ReattachResult {
   scrollback: Uint8Array;
 }
 
+/** A UI subscription attached to one background task output stream. */
+export interface BackgroundTaskAttachment {
+  /** Attached task id, echoed by the backend. */
+  taskId: string;
+  /** Retained bytes to repaint immediately before live chunks arrive. */
+  scrollback: Uint8Array;
+  /** Whether subsequent live bytes are expected on the supplied callback. */
+  live: boolean;
+  /** Detaches the local UI subscriber without cancelling the task. */
+  detach(): void;
+}
+
 /** Projects: create/open/close/list (L2). */
 export interface ProjectGateway {
   /** Lists the projects known to the registry. */
@@ -932,6 +944,15 @@ export interface PermissionGateway {
 export interface WorkStateGateway {
   /** Reads the current per-agent live/offline and idle/busy state for a project. */
   getProjectWorkState(projectId: string): Promise<ProjectWorkState>;
+  /**
+   * Attaches a UI output subscriber to a background task. Running tasks replay
+   * PTY scrollback and then stream live chunks; terminal tasks return only their
+   * persisted output tail.
+   */
+  attachBackgroundTask(
+    taskId: string,
+    onData: (bytes: Uint8Array) => void,
+  ): Promise<BackgroundTaskAttachment>;
   /**
    * Cancels a running/pending background task by id. The read-model refreshes
    * through the `backgroundTaskChanged` domain event.