feat(domain): modèle de tâches de fond de 1re classe et mailbox (B1)

Entités BackgroundTask et Inbox, identifiants, événements et ports du
domaine pour la récupération de complétion post-tour et la réception de
messages concurrents. Fondations des lots infra/application suivants.

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

crates/domain/src/events.rs

@@ -1,7 +1,7 @@
 //! Domain events published on the [`crate::ports::EventBus`] and relayed to the
 //! presentation layer (ARCHITECTURE §3.2).
 
-use crate::ids::{AgentId, ProfileId, ProjectId, SessionId, SkillId, TemplateId};
+use crate::ids::{AgentId, ProfileId, ProjectId, SessionId, SkillId, TaskId, TemplateId};
 use crate::mailbox::TicketId;
 use crate::memory::MemorySlug;
 use crate::template::TemplateVersion;
@@ -41,6 +41,108 @@ pub enum DomainEvent {
         /// The session it runs in.
         session_id: SessionId,
     },
+    /// A first-class background task started running.
+    BackgroundTaskStarted {
+        /// The owning project.
+        project_id: ProjectId,
+        /// The task.
+        task_id: TaskId,
+        /// The agent that owns completion delivery.
+        owner_agent_id: AgentId,
+    },
+    /// A first-class background task changed state.
+    BackgroundTaskStateChanged {
+        /// The owning project.
+        project_id: ProjectId,
+        /// The task.
+        task_id: TaskId,
+        /// The agent that owns completion delivery.
+        owner_agent_id: AgentId,
+        /// New state.
+        state: crate::background_task::BackgroundTaskState,
+    },
+    /// A first-class background task completed successfully.
+    BackgroundTaskCompleted {
+        /// The owning project.
+        project_id: ProjectId,
+        /// The task.
+        task_id: TaskId,
+        /// The agent that owns completion delivery.
+        owner_agent_id: AgentId,
+    },
+    /// A first-class background task failed.
+    BackgroundTaskFailed {
+        /// The owning project.
+        project_id: ProjectId,
+        /// The task.
+        task_id: TaskId,
+        /// The agent that owns completion delivery.
+        owner_agent_id: AgentId,
+    },
+    /// A first-class background task was cancelled.
+    BackgroundTaskCancelled {
+        /// The owning project.
+        project_id: ProjectId,
+        /// The task.
+        task_id: TaskId,
+        /// The agent that owns completion delivery.
+        owner_agent_id: AgentId,
+    },
+    /// A first-class background task has a terminal result not yet delivered.
+    BackgroundTaskCompletionDeliveryPending {
+        /// The owning project.
+        project_id: ProjectId,
+        /// The task.
+        task_id: TaskId,
+        /// The agent that owns completion delivery.
+        owner_agent_id: AgentId,
+    },
+    /// A first-class background task completion was delivered.
+    BackgroundTaskCompletionDelivered {
+        /// The owning project.
+        project_id: ProjectId,
+        /// The task.
+        task_id: TaskId,
+        /// The agent that owns completion delivery.
+        owner_agent_id: AgentId,
+    },
+    /// An item was queued in an agent inbox.
+    AgentInboxQueued {
+        /// Target agent.
+        agent_id: AgentId,
+        /// Queue depth after enqueue.
+        depth: usize,
+    },
+    /// An item was drained from an agent inbox.
+    AgentInboxDrained {
+        /// Target agent.
+        agent_id: AgentId,
+        /// Queue depth after drain.
+        depth: usize,
+    },
+    /// A wake was scheduled for an agent.
+    AgentWakeScheduled {
+        /// Owning project.
+        project_id: ProjectId,
+        /// Target agent.
+        agent_id: AgentId,
+    },
+    /// A wake turn started for an agent.
+    AgentWakeStarted {
+        /// Owning project.
+        project_id: ProjectId,
+        /// Target agent.
+        agent_id: AgentId,
+    },
+    /// A wake attempt failed.
+    AgentWakeFailed {
+        /// Owning project.
+        project_id: ProjectId,
+        /// Target agent.
+        agent_id: AgentId,
+        /// Human-readable reason.
+        reason: String,
+    },
     /// A target agent produced a synchronous reply to an inter-agent `ask`
     /// (ARCHITECTURE §17.4): the requester sent a task via `agent.message`, IdeA
     /// drove the target's structured session to its turn `Final`, and this is the