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/loader.test.ts

@@ -98,7 +98,7 @@ describe("loadPlugins", () => {
     );
     expect(failures).toEqual([]);
     expect((globalThis as Record<string, unknown>).__registerError).toMatch(
-      /not declared by any menu item/,
+      /not declared by any command contribution/,
     );
   });
 
@@ -591,6 +591,13 @@ describe("loadPlugins", () => {
                 command: "hello-plugin",
               },
             ],
+            slashCommands: [
+              {
+                name: "/hello",
+                shortDescription: "Run the hello-plugin callback",
+                command: "hello-plugin",
+              },
+            ],
             layouts: [
               {
                 type: "hello-plugin.hello-world",
@@ -613,6 +620,13 @@ describe("loadPlugins", () => {
       },
     ]);
     expect(registry.get("com.example.hello-plugin")?.contributes.mcpServers).toEqual([]);
+    expect(registry.get("com.example.hello-plugin")?.contributes.slashCommands).toEqual([
+      {
+        name: "/hello",
+        shortDescription: "Run the hello-plugin callback",
+        command: "hello-plugin",
+      },
+    ]);
     await registry.runCommand("com.example.hello-plugin", "hello-plugin");
     expect((globalThis as Record<string, unknown>).__helloArchiveCommandRan).toBe(true);
     const Layout = registry.layoutComponent(
@@ -623,6 +637,39 @@ describe("loadPlugins", () => {
     expect((Layout as unknown as () => string)()).toBe("hello-world");
   });
 
+  it("allows command callbacks declared only by slashCommands", async () => {
+    const bundle = dataUrl(`
+      export function activate(ctx) {
+        ctx.commands.registerCommand("dev.acme.explain", () => {
+          globalThis.__slashOnlyCommandRan = true;
+        });
+      }
+    `);
+    const { registry, failures } = await loadPlugins(
+      [
+        entry({
+          id: "dev.acme.slash",
+          displayName: "Slash",
+          bundleUrl: bundle,
+          contributes: {
+            slashCommands: [
+              {
+                name: "/explain",
+                shortDescription: "Explain selection",
+                command: "dev.acme.explain",
+              },
+            ],
+          } as unknown as PluginContributionDto,
+        }),
+      ],
+      gateways,
+    );
+
+    expect(failures).toEqual([]);
+    await registry.runCommand("dev.acme.slash", "dev.acme.explain");
+    expect((globalThis as Record<string, unknown>).__slashOnlyCommandRan).toBe(true);
+  });
+
   it("confines a malformed runtime catalog entry and still loads healthy plugins", async () => {
     const bundle = dataUrl(`
       export function activate(ctx) {