fix: place orchestrator prompt after AGENTS.md, add ordering test

- Fix leading \n\n artifact when system[0] is empty (use conditional instead of || '')
- Add regression test for orchestrator prompt ordering relative to AGENTS.md

PR #782 thermo review: two REQUIRED fixes applied.

src/index.ts

@@ -1139,9 +1139,9 @@ const OhMyOpenCodeLite: Plugin = async (ctx) => {
             typeof orchestratorDef?.config?.prompt === 'string'
               ? orchestratorDef.config.prompt
               : buildOrchestratorPrompt(disabledAgents);
-          output.system[0] =
-            (output.system[0] || '') +
-            `\n\n${orchestratorPrompt}`;
+          output.system[0] = output.system[0]
+            ? `${output.system[0]}\n\n${orchestratorPrompt}`
+            : orchestratorPrompt;
         }
       }
 

src/system-transform-ordering.test.ts

@@ -0,0 +1,71 @@
+import { describe, expect, test } from 'bun:test';
+import { collapseSystemInPlace } from './utils/system-collapse';
+
+/**
+ * Regression tests for orchestrator prompt ordering in the
+ * experimental.chat.system.transform hook (PR #782).
+ *
+ * The hook places the orchestrator prompt AFTER AGENTS.md so the user's
+ * behavioral rules retain their intended priority. These tests exercise
+ * the core ordering logic that the hook performs.
+ */
+
+const ORCHESTRATOR_PROMPT = 'You are the orchestrator agent.';
+
+function applySystemTransform(
+  system: string[],
+  agentName: string | undefined,
+  orchestratorPrompt = ORCHESTRATOR_PROMPT,
+): void {
+  if (agentName === 'orchestrator') {
+    const alreadyInjected = system.some(
+      (s) =>
+        typeof s === 'string' &&
+        s.includes('<Role>') &&
+        s.includes('orchestrator'),
+    );
+    if (!alreadyInjected) {
+      system[0] = system[0]
+        ? `${system[0]}\n\n${orchestratorPrompt}`
+        : orchestratorPrompt;
+    }
+  }
+  collapseSystemInPlace(system);
+}
+
+describe('system.transform hook ordering (PR #782)', () => {
+  test('orchestrator prompt appears AFTER AGENTS.md content', () => {
+    const system = ['AGENTS.md: always use TypeScript'];
+    applySystemTransform(system, 'orchestrator');
+
+    expect(system).toHaveLength(1);
+    expect(system[0]).toBe(
+      'AGENTS.md: always use TypeScript\n\n' + ORCHESTRATOR_PROMPT,
+    );
+  });
+
+  test('empty system array gets orchestrator prompt only', () => {
+    const system: string[] = [];
+    applySystemTransform(system, 'orchestrator');
+
+    expect(system).toHaveLength(1);
+    expect(system[0]).toBe(ORCHESTRATOR_PROMPT);
+  });
+
+  test('non-orchestrator agent is not modified', () => {
+    const system = ['AGENTS.md: always use TypeScript'];
+    applySystemTransform(system, 'explorer');
+
+    expect(system).toHaveLength(1);
+    expect(system[0]).toBe('AGENTS.md: always use TypeScript');
+  });
+
+  test('already-injected orchestrator prompt is not duplicated', () => {
+    const system = ['<Role> orchestrator </Role>'];
+    applySystemTransform(system, 'orchestrator');
+
+    expect(system).toHaveLength(1);
+    // No double-injection
+    expect(system[0]).toBe('<Role> orchestrator </Role>');
+  });
+});