Gated batch runners: re-check preconditions when the gate opens, not at launch

A runner that checks free memory, disk space and maintenance windows once at start-up, then waits hours for a gate file (a lock, a slot marker), acts on stale checks. Move every precondition check to the moment the gate appears, immediately before taking the lock. Give the wait a hard deadline that exits cleanly (e.g. before a known nightly reboot) instead of firing into the window. Two related traps: (1) if the runner refuses after the gate opens but before its cleanup trap is installed, it leaves the gate file in place and the slot looks taken, so install the trap first or report the refusal so the scheduler can reassign the slot; (2) when testing in a container, stub the external tools and assert that every output row has real numeric fields and the right labels, since bash -n catches neither stale checks nor a comment that silently swallows the rest of a one-line command.