feat(cli): menu contextuel d'autocomplétion des slash-commands dans le composer — #163 (QA verte)

À la saisie d'un « / » en début de draft (sans espace), ouverture d'un menu
contextuel listant les commandes matchant le préfixe, via le contrat unifié
listSlashCommands — aucune liste codée en dur côté UI. Rafinement incrémental
des suggestions à la frappe, navigation clavier + sélection, exécution par
executeSlashCommand.

- adapters/agent: listSlashCommands(prefix?) + executeSlashCommand (invoke Tauri).
- domain + ports: types SlashCommand / ExecuteSlashCommandResult.
- mock: données de test.
- CustomAgentChatView: détection de préfixe (/ sans espace), fetch avec séquence
  anti-retard, gestion ouverture/active index, navigation clavier, fermeture
  quand le préfixe cesse d'être éligible.

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

frontend/src/adapters/agent.test.ts

@@ -203,6 +203,56 @@ describe("TauriAgentGateway invoke payloads", () => {
     });
   });
 
+  it("listSlashCommands forwards the unified slash-command filter request and unwraps commands", async () => {
+    invoke.mockResolvedValueOnce({
+      commands: [
+        {
+          name: "/clean",
+          shortDescription: "Clean current conversation",
+          requiresConfirmation: false,
+          availability: { status: "available" },
+          source: "native",
+          native: "clean",
+        },
+      ],
+    });
+
+    const out = await new TauriAgentGateway().listSlashCommands("/c");
+
+    expect(invoke).toHaveBeenCalledWith("list_slash_commands", {
+      request: { prefix: "/c" },
+    });
+    expect(out).toHaveLength(1);
+    expect(out[0].name).toBe("/clean");
+  });
+
+  it("executeSlashCommand wraps the selected command and current session id", async () => {
+    invoke.mockResolvedValueOnce({
+      command: {
+        name: "/clean",
+        shortDescription: "Clean current conversation",
+        requiresConfirmation: false,
+        availability: { status: "available" },
+        source: "native",
+        native: "clean",
+      },
+      effect: {
+        kind: "cleanConversation",
+        sessionId: "chat-session-1",
+        clearedChunks: 2,
+      },
+    });
+
+    const out = await new TauriAgentGateway().executeSlashCommand("/clean", {
+      sessionId: "chat-session-1",
+    });
+
+    expect(invoke).toHaveBeenCalledWith("execute_slash_command", {
+      request: { name: "/clean", sessionId: "chat-session-1" },
+    });
+    expect(out.effect.kind).toBe("cleanConversation");
+  });
+
   it("launchAgentChat returns a handle only when launch_agent confirms cellKind chat", async () => {
     invoke.mockResolvedValueOnce({
       sessionId: "chat-session-1",