Fixes#1802
The v0.3.0 refactor split child agent construction (_build_child_agent)
and execution (_run_single_child) into separate functions. This created
a scope bug where _saved_tool_names was defined in _build_child_agent
but referenced in _run_single_child's finally block, causing a NameError
on every delegate_task call.
Solution: Move the save/restore logic entirely into _run_single_child,
keeping the save and restore in the same scope as the try/finally block.
This is cleaner than passing the variable through and removes the dead
save from _build_child_agent.
ba7248c669