test(todo): exercise parallel in_progress items in the keyless snapshot

The recorded ACP todo-write scenario still submitted one active item, so
no keyless assembled check exercised the new parallel contract. Re-record
the scenario with a three-step plan carrying two simultaneously
in_progress tasks; the replayed todo/write event now pins the parallel
shape without a key.

examples/acp-agent/tests/snapshots/todo-write/input.json

@@ -2,6 +2,6 @@
   "steps": [
     { "op": "initialize" },
     { "op": "newSession" },
-    { "op": "prompt", "text": "Use the todo_write tool to record a plan with exactly three todos: \"read the code\" (in_progress), \"write the fix\" (pending), \"run the tests\" (pending). Send all three in one todo_write call. Then reply with the single word DONE and stop." }
+    { "op": "prompt", "text": "Use the todo_write tool to record a plan with exactly three todos for work running in parallel: \"read the code\" (in_progress), \"watch the background build\" (in_progress), \"write the fix\" (pending). Send all three in one todo_write call. Then reply with the single word DONE and stop." }
   ]
 }