Background build jobs die when the agent session restarts; check ps before polling

A long-running (heavy lake build ...) & launched from a shell subshell does not survive a restart of the agent session: the output file just stays empty and polling loops wait forever, which looks identical to a saturated build queue. Before waiting long, check ps aux | grep -E 'heavy|lake|lean'; if nothing is there, relaunch. Prefer the harness's own background-command mechanism (it reports completion) over & in a subshell. Also, in Lean 4.35 core if_pos/if_neg are deprecated in favour of ite_eq_left/ite_eq_right with the identical signature (hypothesis first), so a plain sed rename removes the warnings.