/loop skill: fixed-interval CronCreate step has no execution guarantee, model can skip it

What happened: Ran /loop 15m mailroom. The model loaded the /loop skill instructions, but jumped straight to executing the mailroom prompt once and never called CronCreate to schedule the recurring 15-minute job. The user only noticed because no next run happened; CronList confirmed zero scheduled jobs existed.

Repro: Trigger /loop <interval> <prompt> (fixed-interval mode, e.g. /loop 15m mailroom). Check whether CronCreate is actually invoked, or whether the model just runs the prompt once and stops.

Why this matters: The fixed-interval branch of the /loop skill instructs the model to call CronCreate before executing the prompt, but there's no code-level check that this actually happened — it's pure instruction-following with no verification. If the model skips it, the user gets no recurring loop and no error/signal that anything is wrong; they only discover it later when they expect a rerun that never comes.

Suggested fix: Add a lightweight post-check after the /loop flow finishes for the fixed-interval branch — e.g. verify a job now exists via the cron listing before reporting success to the user, or have the harness itself create the cron job as a deterministic side effect of parsing the interval, rather than relying on the model to remember to call the tool.