feat: support compact command across IM channels

plugin-src/host/harness-command-executor.mjs

@@ -0,0 +1,21 @@
+/**
+ * Adapt the Host's Typert command gateway to the channel Harness client.
+ * Programmatic production fixtures may omit the gateway and exercise other assembly paths.
+ */
+export function createHarnessCommandExecutor(ctx, provided) {
+  if (provided !== undefined) {
+    if (typeof provided !== 'function') throw new TypeError('commandExecutor must be a function');
+    return provided;
+  }
+  const gateway = ctx?.typertGateway;
+  if (!gateway) return undefined;
+  if (typeof gateway.invoke !== 'function') {
+    throw new TypeError('dsh-im requires a callable ctx.typertGateway');
+  }
+  return (sessionId, line, options = {}) => gateway.invoke({
+    namespace: 'commands',
+    method: 'execute',
+    args: { agentId: sessionId, line },
+    signal: options.signal,
+  });
+}