fix(exec): terminate one-shot process trees on cleanup

Run one-shot commands in their own process tree and terminate all
descendants after timeout, cancellation, or unexpected failures.

Co-authored-by: TRAE CLI <noreply@bytedance.com>

nanobot/agent/tools/shell.py

@@ -326,6 +326,7 @@ class ExecTool(Tool):
                 prepared.env,
                 prepared.shell_program,
                 prepared.login,
+                process_tree=True,
             )
 
             try:
@@ -334,10 +335,10 @@ class ExecTool(Tool):
                     timeout=prepared.timeout,
                 )
             except asyncio.TimeoutError:
-                await self._kill_process(process)
+                await self._kill_process_tree(process)
                 return ToolResult.error(f"Error: Command timed out after {prepared.timeout} seconds")
             except asyncio.CancelledError:
-                await self._kill_process(process)
+                await self._kill_process_tree(process)
                 raise
 
             # Safety-net reap: asyncio *should* have reaped the child via
@@ -374,7 +375,7 @@ class ExecTool(Tool):
             # Kill and reap the child if it was spawned but an unexpected
             # error prevented communicate() from completing.
             if process is not None:
-                await self._kill_process(process)
+                await self._kill_process_tree(process)
             return ToolResult.error(f"Error executing command: {str(e)}")
 
     async def _execute_session(