feat: livrable ticket #91 — notification fin BackgroundTask enrichie

- Backend: enrichissement HeadlessRendezvous avec requester/target/conversationId
- Frontend: toast 'Requester -> Target completed/...' explicite
- Clic ouvrant viewer de conversation pour visualiser l'échange

crates/application/src/workstate/mod.rs

@@ -231,6 +231,12 @@ pub struct AgentBackgroundTaskState {
     pub stdout_tail: Option<String>,
     /// Bounded stderr tail.
     pub stderr_tail: Option<String>,
+    /// Agent that requested a headless rendezvous, when this task is one.
+    pub requester_agent_id: Option<AgentId>,
+    /// Target agent for a headless rendezvous, when this task is one.
+    pub target_agent_id: Option<AgentId>,
+    /// Conversation opened by a headless rendezvous, when this task is one.
+    pub conversation_id: Option<domain::ConversationId>,
     /// Creation timestamp, epoch milliseconds.
     pub created_at_ms: u64,
     /// Last update timestamp, epoch milliseconds.
@@ -592,6 +598,8 @@ impl GetProjectWorkState {
 impl From<BackgroundTask> for AgentBackgroundTaskState {
     fn from(task: BackgroundTask) -> Self {
         let (exit_code, summary, stdout_tail, stderr_tail) = flatten_background_result(&task);
+        let (requester_agent_id, target_agent_id, conversation_id) =
+            flatten_background_rendezvous_context(&task.kind);
         Self {
             task_id: task.id,
             kind: BackgroundTaskKindLabel::from(&task.kind),
@@ -600,12 +608,37 @@ impl From<BackgroundTask> for AgentBackgroundTaskState {
             summary,
             stdout_tail,
             stderr_tail,
+            requester_agent_id,
+            target_agent_id,
+            conversation_id,
             created_at_ms: task.created_at_ms,
             updated_at_ms: task.updated_at_ms,
         }
     }
 }
 
+fn flatten_background_rendezvous_context(
+    kind: &BackgroundTaskKind,
+) -> (
+    Option<AgentId>,
+    Option<AgentId>,
+    Option<domain::ConversationId>,
+) {
+    match kind {
+        BackgroundTaskKind::HeadlessRendezvous {
+            requester_agent_id,
+            target_agent_id,
+            conversation_id,
+            ..
+        } => (
+            *requester_agent_id,
+            Some(*target_agent_id),
+            Some(*conversation_id),
+        ),
+        _ => (None, None, None),
+    }
+}
+
 impl From<&BackgroundTaskKind> for BackgroundTaskKindLabel {
     fn from(kind: &BackgroundTaskKind) -> Self {
         match kind {