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/plugins/runtime/registry.ts

@@ -59,7 +59,7 @@ export interface Disposable {
   dispose(): void;
 }
 
-export type PluginCommandHandler = (...args: unknown[]) => void | Promise<void>;
+export type PluginCommandHandler = (...args: unknown[]) => unknown | Promise<unknown>;
 
 /** Commands a plugin registers in `activate(ctx)`, dispatched by menu items. */
 export class PluginCommandRegistry {
@@ -74,7 +74,7 @@ export class PluginCommandRegistry {
     if (!this.declaredCommandIds.has(commandId)) {
       throw new Error(
         `plugin "${this.pluginId}" tried to register command "${commandId}" ` +
-          "which is not declared by any menu item in its manifest",
+          "which is not declared by any command contribution in its manifest",
       );
     }
     this.handlers.set(commandId, handler);
@@ -86,11 +86,11 @@ export class PluginCommandRegistry {
   }
 
   /** Runs a registered command; a no-op (never throws) if none is registered. */
-  async run(commandId: string, ...args: unknown[]): Promise<void> {
+  async run(commandId: string, ...args: unknown[]): Promise<unknown> {
     const handler = this.handlers.get(commandId);
     if (!handler) return;
     try {
-      await handler(...args);
+      return await handler(...args);
     } catch (e) {
       console.error(
         `[plugin:${this.pluginId}] command "${commandId}" failed`,
@@ -99,6 +99,17 @@ export class PluginCommandRegistry {
     }
   }
 
+  /** Runs a registered command and surfaces failures to the caller. */
+  async runStrict(commandId: string, ...args: unknown[]): Promise<unknown> {
+    const handler = this.handlers.get(commandId);
+    if (!handler) {
+      throw new Error(
+        `plugin "${this.pluginId}" has no registered command handler "${commandId}"`,
+      );
+    }
+    return await handler(...args);
+  }
+
   has(commandId: string): boolean {
     return this.handlers.has(commandId);
   }
@@ -256,7 +267,19 @@ export class PluginRuntimeRegistry {
     );
   }
 
-  async runCommand(pluginId: string, commandId: string, ...args: unknown[]): Promise<void> {
-    await this.loaded.get(pluginId)?.commands.run(commandId, ...args);
+  async runCommand(pluginId: string, commandId: string, ...args: unknown[]): Promise<unknown> {
+    return await this.loaded.get(pluginId)?.commands.run(commandId, ...args);
+  }
+
+  async runCommandStrict(
+    pluginId: string,
+    commandId: string,
+    ...args: unknown[]
+  ): Promise<unknown> {
+    const plugin = this.loaded.get(pluginId);
+    if (!plugin) {
+      throw new Error(`plugin "${pluginId}" is not loaded`);
+    }
+    return await plugin.commands.runStrict(commandId, ...args);
   }
 }