fix(benchmarks): kill timed-out agent cells cross-platform (#225)

run_cell's timeout handler called `taskkill /F /T`, a Windows-only command, with
no POSIX branch. On macOS/Linux a timed-out cell raised FileNotFoundError instead
of being killed, leaking the agent's process tree and logging the cell as an error
rather than a clean timeout-kill.

Branch the kill on platform: taskkill on Windows, os.killpg on POSIX. The cell now
starts in its own session (Popen start_new_session) so its process group is
killable. A _selftest_kill check (run.py --selftest) spawns a sleeper and asserts
the tree-kill terminates it on whatever platform CI runs.