feat(agent): messagerie inter-agents via send_blocking (D6) — §17

Comble le trou historique : un agent qui en interroge un autre reçoit enfin
le CONTENU de la réponse, plus seulement un ACK de cycle de vie.

- domain : OrchestratorCommand::AskAgent { target, task } + action wire
  agent.message (target+task requis) ; DomainEvent::AgentReplied
  { agent_id, reply_len } (bus I/O-free, le contenu remonte par l'outcome).
- application : OrchestratorOutcome gagne reply: Option<String>. Méthode
  ask_agent (§17.4) — cible structurée vivante ⇒ send_blocking direct ;
  cible morte ⇒ LaunchAgent structuré puis send ; agent vivant en PTY ou
  profil sans structured_adapter ⇒ Invalid (non adressable, jamais d'ACK
  trompeur) ; agent inconnu ⇒ NotFound ; service non câblé ⇒ Invalid.
  Timeout (300s) ⇒ erreur typée SANS tuer la session. Succès ⇒ publie
  AgentReplied + reply: Some(content). Injection additive via builders
  with_structured / with_events (call sites legacy intacts).
- app-tauri : state câble with_structured/with_events ; DTO AgentReplied.

Tests (QA) : +14 verts (11 app + 3 domaine), invariants validés par
mutation test (reply!=None, timeout ne shutdown pas). cargo test
--workspace : 817 passed, 0 failed.

Suivi (D6b) : surfacer reply dans le writer wire .response.json
(infrastructure/orchestrator) pour la délégation par protocole fichier.
Reste D7 : menu restreint Claude/Codex + retrait custom.

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

crates/domain/src/events.rs

@@ -25,6 +25,18 @@ pub enum DomainEvent {
         /// The session it runs in.
         session_id: SessionId,
     },
+    /// 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
+    /// observability beacon for that completed rendezvous. Carries the target id and
+    /// the reply size (a lightweight metric); the full body is returned to the
+    /// requester out-of-band, not in the event payload (the bus stays I/O-free).
+    AgentReplied {
+        /// The agent that produced the reply.
+        agent_id: AgentId,
+        /// Number of bytes in the reply content (preview metric, not the payload).
+        reply_len: usize,
+    },
     /// An agent's process exited.
     AgentExited {
         /// The agent.