feat: add Pi subagent scheduler and child runtime

tests/unit/pi-rpc-foundation.test.ts

@@ -170,6 +170,20 @@ describe('Pi worker process', () => {
       'read,bash,edit,write,grep,find,ls,ask_user',
       '--model', 'model-a',
     ]);
+    expect(buildPiRpcArgs('sessions', ['--no-session'], ['read', 'grep', 'find', 'ls']))
+      .toEqual([
+        '--mode', 'rpc',
+        '--offline',
+        '--session-dir', 'sessions',
+        '--no-extensions',
+        '--no-skills',
+        '--no-prompt-templates',
+        '--no-themes',
+        '--no-context-files',
+        '--no-approve',
+        '--tools', 'read,grep,find,ls',
+        '--no-session',
+      ]);
   });
 
   it('correlates out-of-order responses, dispatches events, and reassembles partial lines', async () => {