fix: include result preview in child completion instruction

The instruction field is what actually reaches the parent through
the inbox→execution engine path. Previously it only said "check
its output with get_output()" with no preview, forcing an extra
tool call round-trip every time. Now the first 2000 chars of the
child's output are included inline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: EightRice

atn/runtime/agent_registry.py

@@ -287,6 +287,7 @@ async def on_agent_completed(
                 "instruction": (
                     f"Your child agent '{defn.name}' has {status_str}. "
                     f"Check its output with get_output('{agent_id}')."
+                    + (f"\n\nResult summary:\n{result_preview}" if result_preview else "")
                 ),
             },
         )