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/tests/error_codes.rs

@@ -3,7 +3,8 @@
 
 use application::AppError;
 use domain::ports::{
-    EmbedderError, FsError, GitError, MemoryError, ProcessError, PtyError, RemoteError, StoreError,
+    AgentSessionError, EmbedderError, FsError, GitError, MemoryError, ProcessError, PtyError,
+    RemoteError, StoreError,
 };
 
 #[test]
@@ -52,6 +53,31 @@ fn process_pty_runtime_map_to_process() {
     );
 }
 
+#[test]
+fn agent_session_errors_all_map_to_process() {
+    // §17.1 / D4: a structured AgentSession is a live process/SDK conversation, so
+    // every failure variant folds into PROCESS (the byte-stream twin). Used by the
+    // agent_send / close_agent_session command error arms.
+    assert_eq!(
+        AppError::from(AgentSessionError::Start("no cli".into())).code(),
+        "PROCESS"
+    );
+    assert_eq!(
+        AppError::from(AgentSessionError::Io("broken pipe".into())).code(),
+        "PROCESS"
+    );
+    assert_eq!(
+        AppError::from(AgentSessionError::Decode("bad schema".into())).code(),
+        "PROCESS"
+    );
+    assert_eq!(AppError::from(AgentSessionError::Timeout).code(), "PROCESS");
+    // The message is carried through onto the single Process shape.
+    assert_eq!(
+        AppError::from(AgentSessionError::Io("broken pipe".into())),
+        AppError::Process(AgentSessionError::Io("broken pipe".into()).to_string())
+    );
+}
+
 #[test]
 fn git_and_remote_map_through() {
     assert_eq!(AppError::from(GitError::NotFound).code(), "GIT");