refactor(agent-loop): simplify message machine

examples/headless-agent/tests/semantic-checkpoint.snapshot.ts

@@ -32,7 +32,7 @@ async function seedInterruptedSession(root: string, cwd: string): Promise<string
     delegationDepth: 0,
   }
   const events: SessionEvent[] = [
-    { type: 'turn/start', seq: 0, time: 10, data: { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } } },
+    { type: 'turn/start', seq: 0, time: 10, data: { turn: 1 } },
     { type: 'user/message', seq: 1, time: 11, data: createUserMessage({
       content: [{ type: 'text', text: 'Perform one side-effecting remote mutation.' }], source: { kind: 'user' },
     }), surfaceOp: 'append' },