test(todo): align real-model e2e prompt and scenario with parallel todos

The key-gated todo_write e2e still pinned the previous contract: its
TODO_SYSTEM_PROMPT instructed at most one in_progress task and the
scenario never exercised parallel active tasks. Update the prompt to the
new guidance and make the scenario record two simultaneously in_progress
tasks; verified against the real API.

examples/headless-agent/tests/harness.ts

@@ -28,8 +28,9 @@ export const SYSTEM_PROMPT = 'You are a coding agent. Use bash for file operatio
 /** System prompt for the todo_write e2e: nudges the model to plan with the tool. */
 export const TODO_SYSTEM_PROMPT = 'You are a coding agent. For multi-step work, '
   + 'use the todo_write tool to track a task list: send the WHOLE list each call, '
-  + 'keep at most one task in_progress (exactly one while work remains), and mark '
-  + 'a task completed as soon as it is done.'
+  + 'mark every task being actively worked on in_progress (several at once when '
+  + 'work runs in parallel, at least one while work remains), and mark a task '
+  + 'completed as soon as it is done.'
 
 /** Options for {@link codingHarness}. */
 export interface CodingHarnessOptions {