feat(agent): commandes Tauri + ChatBridge streaming (D4) — §17

Expose l'exécution structurée à l'UI (jumeau du chemin PTY) :
- ChatBridge (chat.rs) : pompe ReplyStream -> Channel<ReplyChunk>,
  generation-tracked (anti-double-pompe) ; scrollback de conversation
  côté transport (le port AgentSession est sans mémoire), purgé à la
  fermeture, préservé à la ré-attache.
- Commandes agent_send / reattach_agent_chat / close_agent_session.
- DTO : ReplyChunk (textDelta/toolActivity/final, camelCase tagué kind),
  ReattachChatDto, CellKind {pty,chat} + champ cellKind dérivé sur
  TerminalSessionDto (chat ssi LaunchAgentOutput.structured = Some).
- Wiring composition root : StructuredSessions + ChatBridge dans AppState,
  via la factory déjà injectée (aucun new d'adapter — règle D §17.8).
- From<AgentSessionError> for AppError (mappe sur PROCESS).

Tests (QA) : 35 unitaires verts — generation supersede validé par mutation
test, séquence deltas*+Final, reattach sans re-spawn, teardown shutdown+
unregister, DTO cellKind + round-trip ReplyChunk, non-régression PTY.
cargo test --workspace : 803 passed, 0 failed.

Reste D5 : AgentChatView (frontend) + routage cellKind dans LayoutGrid.

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

crates/application/src/error.rs

@@ -6,8 +6,8 @@
 //! with one error shape when building its `ErrorDTO`.
 
 use domain::ports::{
-    EmbedderError, FsError, GitError, MemoryError, ProcessError, PtyError, RemoteError,
-    RuntimeError, StoreError,
+    AgentSessionError, EmbedderError, FsError, GitError, MemoryError, ProcessError, PtyError,
+    RemoteError, RuntimeError, StoreError,
 };
 use domain::{AgentId, NodeId};
 
@@ -148,3 +148,15 @@ impl From<RemoteError> for AppError {
         Self::Remote(e.to_string())
     }
 }
+
+impl From<AgentSessionError> for AppError {
+    /// Maps a structured [`AgentSessionError`] (ARCHITECTURE §17.1) onto the single
+    /// application error shape. `Start`/`Io`/`Decode`/`Timeout` are all execution
+    /// failures of a live structured session (a process/SDK conversation), so they
+    /// fold into [`AppError::Process`] — coherent with [`PtyError`]/[`ProcessError`],
+    /// the byte-stream twins. The raw CLI JSON never travels through `Decode` (the
+    /// adapter already redacted it), so this is safe to surface.
+    fn from(e: AgentSessionError) -> Self {
+        Self::Process(e.to_string())
+    }
+}