chengyongru cbc8877aa2 feat(long-task): improve prompt structure and agent contract
- Expand LongTaskTool.description to instruct parent agent on goal
  construction, return value semantics, and how to handle results.
- Expand CompleteTool.description to emphasize that the summary IS the
  final answer returned to the parent agent.
- Prefix validated return value with an explicit "final answer" directive
  to stop parent agent from re-running work.
- Redesign step_start.md: Step 1 is now explicitly for exploration,
  planning, and skeleton-building. complete() is discouraged.
- Remove bulky payload debug logging from _emit(); add targeted
  info/warning/error logs at key state transitions instead.
- Add signal_type to HandoffState for cleaner signal detection.
2026-05-14 01:06:05 +08:00
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