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/infrastructure/src/lib.rs

@@ -38,9 +38,9 @@ pub mod store;
 pub mod timeparse;
 
 pub use background_task::{
-    start_background_ready_inbox_bridge, BackgroundCompletionSink, BackgroundCompletionSinkError,
-    BackgroundCompletionSinkOutcome, BackgroundReadyInboxBridgeHandle,
-    BackgroundTaskReadyToDeliver,
+    bounded_tail, start_background_ready_inbox_bridge, tail_cap_bytes, BackgroundCompletionSink,
+    BackgroundCompletionSinkError, BackgroundCompletionSinkOutcome, BackgroundReadyInboxBridgeHandle,
+    BackgroundTaskReadyToDeliver, BoundedTail, CommandBackgroundRunner,
 };
 pub use clock::SystemClock;
 pub use conversation::InMemoryConversationRegistry;