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>

crates/app-tauri/src/commands.rs

@@ -30,6 +30,7 @@ use application::{
     UpdateProjectMcpToolPermissionsInput, UpdateProjectPermissionsInput,
     UpdateProjectSystemPermissionsInput, UpdateSkillInput,
 };
+use backend::stream::OutputSink;
 use domain::ports::ModelServerRuntime;
 use domain::ports::PtyHandle;
 
@@ -38,9 +39,9 @@ use crate::dto::{
     parse_layout_id, parse_memory_slug, parse_model_server_id, parse_node_id, parse_profile_id,
     parse_project_id, parse_session_id, parse_skill_id, parse_task_id, parse_template_id,
     parse_ticket_id, save_model_server_input, AgentDriftListDto, AgentDto, AgentListDto,
-    AppExitWorkGuardStateDto, AssignSkillRequestDto, AttachLiveAgentRequestDto,
-    AttachLiveAgentResponseDto, BackgroundTaskDto, ChangeAgentProfileDto,
-    ChangeAgentProfileRequestDto, CloneOpenCodeProfileFromSeedRequestDto,
+    AppExitWorkGuardStateDto, AssignSkillRequestDto, AttachBackgroundTaskResultDto,
+    AttachLiveAgentRequestDto, AttachLiveAgentResponseDto, BackgroundTaskDto,
+    ChangeAgentProfileDto, ChangeAgentProfileRequestDto, CloneOpenCodeProfileFromSeedRequestDto,
     CloneProfileFromSeedRequestDto, ConfigureProfilesRequestDto, ConversationDetailsDto,
     CreateAgentFromTemplateRequestDto, CreateAgentRequestDto, CreateLayoutRequestDto,
     CreateLayoutResultDto, CreateMemoryRequestDto, CreateProjectRequestDto, CreateSkillRequestDto,
@@ -77,6 +78,7 @@ use crate::embedded_server::{
 };
 use crate::pty::{PtyBridge, PtyChunk};
 use crate::state::{AppState, FocusedProjectDto};
+use crate::stream::TauriChannelSink;
 use domain::{DeviceId, SkillRef, SkillScope};
 use uuid::Uuid;
 
@@ -3745,6 +3747,54 @@ pub async fn retry_background_task(
         .map_err(ErrorDto::from)
 }
 
+/// `attach_background_task` — attach a UI output subscriber to a command-backed
+/// background task.
+///
+/// If the task is still live, the response contains PTY scrollback and a fresh
+/// live subscription is pumped to `on_output`. If it is already terminal, the
+/// response contains only the persisted output tail and no PTY subscription is
+/// attempted.
+///
+/// # Errors
+/// Returns an [`ErrorDto`] (`INVALID` for malformed id, `NOT_FOUND` if the task
+/// has no live handle and no terminal result, `PROCESS`/`STORE` on backend
+/// failure).
+#[tauri::command]
+pub async fn attach_background_task(
+    task_id: String,
+    on_output: Channel<PtyChunk>,
+    state: State<'_, AppState>,
+) -> Result<AttachBackgroundTaskResultDto, ErrorDto> {
+    let id = parse_task_id(&task_id)?;
+    let plan = state
+        .background_task_attach_plan(id)
+        .await
+        .map_err(ErrorDto::from)?;
+    let live = plan.live_handle.is_some();
+
+    if let Some(handle) = plan.live_handle {
+        match state.pty_port.subscribe_output(&handle) {
+            Ok(stream) => {
+                std::thread::spawn(move || {
+                    let sink = TauriChannelSink::new(on_output);
+                    for chunk in stream {
+                        if sink.send(chunk).is_err() {
+                            break;
+                        }
+                    }
+                });
+            }
+            Err(e) => return Err(ErrorDto::from(AppError::from(e))),
+        }
+    }
+
+    Ok(AttachBackgroundTaskResultDto {
+        task_id: plan.task_id.to_string(),
+        scrollback: plan.scrollback,
+        live,
+    })
+}
+
 /// `list_background_tasks` — read the background-task read-model for a project,
 /// optionally narrowed to one owning agent.
 ///