Fix scheduling pre-step to exit cleanly when no programs are due

[Copilot]

The scheduling pre-step exits with process.exit(1) when no programs are due, causing every idle scheduled run to show as a failed workflow (red X) in the Actions UI. On a 6h schedule, most runs have nothing to do — this is normal, not an error.

Changes

• process.exit(1) → process.exit(0) for the "no programs due" path, matching the existing no_programs: true early-exit pattern
• Added not_due flag to the /tmp/gh-aw/autoloop.json sentinel so the agent step can explicitly distinguish "nothing to do" from "program selected"
• Documented not_due in the agent instructions section alongside the existing JSON contract fields

const notDue = !selected && unconfigured.length === 0;
const result = {
    selected: selected,
    // ...
    no_programs: false,
    not_due: notDue,
};

Error exits (process.exit(1)) for forced-program-not-found, forced-program-unconfigured, and unhandled exceptions are unchanged.