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/mock/index.ts

@@ -79,6 +79,8 @@ import type {
   ServerExposureSettings,
   Skill,
   ReplyChunk,
+  ExecuteSlashCommandResult,
+  SlashCommand,
   SkillScope,
   Sprint,
   Template,
@@ -399,6 +401,35 @@ export class MockAgentGateway implements AgentGateway {
   private liveKindByAgent = new Map<string, "pty" | "structured">();
   /** Retained structured reply chunks per live chat session. */
   private chatScrollback = new Map<string, ReplyChunk[]>();
+  private slashCommands: SlashCommand[] = [
+    {
+      name: "/help",
+      shortDescription: "Afficher les commandes disponibles",
+      requiresConfirmation: false,
+      availability: { status: "available" },
+      source: "native",
+      native: "help",
+    },
+    {
+      name: "/clean",
+      shortDescription: "Nettoyer la conversation courante",
+      requiresConfirmation: false,
+      availability: { status: "available" },
+      source: "native",
+      native: "clean",
+    },
+    {
+      name: "/profile",
+      shortDescription: "Changer le profil de l'agent",
+      requiresConfirmation: true,
+      availability: {
+        status: "unavailable",
+        reason: "La selection de profil est livree par le ticket #164",
+      },
+      source: "native",
+      native: "profile",
+    },
+  ];
 
   private getAgents(projectId: string): Agent[] {
     if (!this.agents.has(projectId)) this.agents.set(projectId, []);
@@ -862,6 +893,61 @@ export class MockAgentGateway implements AgentGateway {
     }
   }
 
+  async listSlashCommands(prefix?: string): Promise<SlashCommand[]> {
+    const normalized = prefix?.trim();
+    const query = normalized
+      ? normalized.startsWith("/")
+        ? normalized
+        : `/${normalized}`
+      : "";
+    return structuredClone(
+      this.slashCommands.filter((command) =>
+        query ? command.name.startsWith(query) : true,
+      ),
+    );
+  }
+
+  async executeSlashCommand(
+    name: string,
+    options: { sessionId?: string | null } = {},
+  ): Promise<ExecuteSlashCommandResult> {
+    const normalized = name.trim().startsWith("/") ? name.trim() : `/${name.trim()}`;
+    const command = this.slashCommands.find((item) => item.name === normalized);
+    if (!command) {
+      throw { code: "NOT_FOUND", message: `slash command ${normalized}` } as GatewayError;
+    }
+    if (command.availability.status === "unavailable") {
+      throw {
+        code: "INVALID",
+        message: `slash command ${normalized} unavailable: ${command.availability.reason}`,
+      } as GatewayError;
+    }
+    if (command.native === "clean") {
+      if (!options.sessionId) {
+        throw {
+          code: "INVALID",
+          message: "/clean requires a current session id",
+        } as GatewayError;
+      }
+      this.chatScrollback.set(options.sessionId, []);
+      return {
+        command: structuredClone(command),
+        effect: {
+          kind: "cleanConversation",
+          sessionId: options.sessionId,
+          clearedChunks: 0,
+        },
+      };
+    }
+    return {
+      command: structuredClone(command),
+      effect: {
+        kind: "help",
+        commands: structuredClone(this.slashCommands),
+      },
+    };
+  }
+
   async closeAgentChat(sessionId: string): Promise<void> {
     this.chatScrollback.delete(sessionId);
     for (const [agentId, liveSessionId] of this.liveSessionByAgent) {