feat(background): runner PTY B8 + boucle sink fermée + refactor point-2

Livre le lot backend B8 des tâches de fond :

- infrastructure : runner de commandes concret (CommandBackgroundRunner)
  sur le port BackgroundTaskRunner, tail borné (bounded_tail/BoundedTail),
  éclatement du module background_task en sous-modules (mod/runner/tail,
  sink extrait de l'ancien background_task.rs).
- application : nouveau module background exposant les cas d'usage
  SpawnBackgroundCommand, CancelBackgroundTask, RetryBackgroundTask et le
  port BackgroundCommandArchive.
- domain : refactor point-2 de l'arbitrage Architect — sortie du trait
  BackgroundCommandArchive de la couche domaine vers application.
- app-tauri : câblage runtime (commands, dto, state, lib) des commandes
  spawn/cancel/retry et de la boucle de complétion sink fermée en
  composition root.

Build workspace + tests application/infrastructure verts (QA).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/domain/src/lib.rs

@@ -167,9 +167,9 @@ pub use orchestrator::{
 };
 
 pub use ports::{
-    AgentContextStore, AgentRuntime, BackgroundCompletionStream, BackgroundTaskCompletion,
-    BackgroundTaskHandle, BackgroundTaskPortError, BackgroundTaskRunner, BackgroundTaskSpec,
-    BackgroundTaskStore, Clock, ContextInjectionPlan, DirEntry, Embedder, EmbedderEnvInspector,
+    AgentContextStore, AgentRuntime, BackgroundCompletionStream,
+    BackgroundTaskCompletion, BackgroundTaskHandle, BackgroundTaskPortError, BackgroundTaskRunner,
+    BackgroundTaskSpec, BackgroundTaskStore, Clock, ContextInjectionPlan, DirEntry, Embedder, EmbedderEnvInspector,
     EmbedderEnvReport, EmbedderError, EmbedderProfileStore, EmbedderPromptDismissal,
     EmbedderPromptStore, EventBus, EventStream, ExitStatus, FileSystem, FsError, GitCommitInfo,
     GitError, GitFileStatus, GitPort, GraphCommit, IdGenerator, LiveStateStore, MemoryError,