feat(cli): dispatch du callback plugin slash-command vers le runtime registry — #166 (QA verte)

Boucle le gap end-to-end de #165 : à l'exécution d'une commande slash plugin
depuis le composer custom, l'UI traite l'effet pluginCallback et le dispatche
au PluginRuntimeRegistry pour que la callback du plugin s'exécute réellement.

- runtime/registry: runCommandStrict (échec explicite si handler absent) +
  propagation de la valeur de retour ; élargit les types de retour (unknown).
- runtime/loader: chargement de contributes.slashCommands au registre +
  propagation du retour du handler.
- features/plugins/usePluginMenus: runCommand retourne unknown.
- features/agents/CustomAgentChatView: handler de l'effet pluginCallback ->
  pluginRuntime.registry.runCommandStrict, avec retour utilisateur en cas
  d'échec (commande indisponible / callback absente).

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

frontend/src/features/agents/CustomAgentChatView.tsx

@@ -26,6 +26,7 @@ import type {
   SlashCommand,
 } from "@/domain";
 import { useGateways } from "@/app/di";
+import { usePluginRuntime } from "@/features/plugins/PluginRuntimeProvider";
 import { Button, Spinner, cn } from "@/shared";
 import type { ChatAttachmentInput } from "@/ports";
 
@@ -66,6 +67,11 @@ interface ProfileCommandDialogState {
   error: string | null;
 }
 
+interface SlashCommandInvocation {
+  name: string;
+  arguments: unknown[];
+}
+
 function describe(e: unknown): string {
   if (e && typeof e === "object" && "message" in e) {
     return String((e as GatewayError).message);
@@ -274,6 +280,17 @@ function slashCommandPrefix(draft: string): string | null {
   return draft;
 }
 
+function slashCommandInvocation(text: string): SlashCommandInvocation | null {
+  if (!text.startsWith("/")) return null;
+  const [name, ...argumentParts] = text.split(/\s+/);
+  if (!name) return null;
+  const rawArguments = argumentParts.join(" ").trim();
+  return {
+    name,
+    arguments: rawArguments ? [rawArguments] : [],
+  };
+}
+
 function isSlashCommandAvailable(command: SlashCommand): boolean {
   return command.availability.status === "available";
 }
@@ -328,6 +345,7 @@ export function CustomAgentChatView({
   onConversationId,
 }: CustomAgentChatViewProps) {
   const { agent, profile: profileGateway, system } = useGateways();
+  const pluginRuntime = usePluginRuntime();
   const [turns, setTurns] = useState<ChatTurn[]>([]);
   const [currentSession, setCurrentSession] = useState(sessionId);
   const [externalSessionId, setExternalSessionId] = useState(sessionId);
@@ -756,6 +774,45 @@ export function CustomAgentChatView({
     }
   }
 
+  async function executeSlashCommand(invocation: SlashCommandInvocation) {
+    if (!agent.executeSlashCommand) {
+      setError(`Commande ${invocation.name} indisponible dans ce runtime.`);
+      return;
+    }
+    setDraft("");
+    setError(null);
+    try {
+      const sid =
+        currentSession ??
+        (await recoverStructuredSession({
+          applyScrollback: false,
+          retryAttachNotFound: true,
+        }));
+      const result = await agent.executeSlashCommand(invocation.name, {
+        sessionId: sid,
+        arguments: invocation.arguments,
+      });
+      if (result.effect.kind !== "pluginCallback") {
+        throw new Error(
+          `La commande ${invocation.name} n'a pas renvoyé d'effet pluginCallback.`,
+        );
+      }
+      await pluginRuntime.registry.runCommandStrict(
+        result.effect.pluginId,
+        result.effect.commandId,
+        ...result.effect.arguments,
+      );
+      setTurns((prev) => [
+        ...prev,
+        { role: "tool", label: `Commande plugin ${result.command.name} exécutée.` },
+      ]);
+    } catch (e) {
+      const message = `Commande ${invocation.name} échouée: ${describe(e)}`;
+      setError(message);
+      setTurns((prev) => [...prev, { role: "error", text: message }]);
+    }
+  }
+
   async function pickAttachment() {
     const path = await system.pickFile();
     if (path) {
@@ -804,6 +861,11 @@ export function CustomAgentChatView({
       await openProfileCommandFlow();
       return;
     }
+    const slashInvocation = slashCommandInvocation(text);
+    if (slashInvocation) {
+      await executeSlashCommand(slashInvocation);
+      return;
+    }
     const outgoingAttachments = attachments;
     const attachmentInputs = outgoingAttachments.map((item) => item.input);
     const attachmentLabels = outgoingAttachments.map((item) => item.label);