Schedule a prompt to fire on a fixed cadence (recurring loop). Use when the user asks to "run X every N minutes/hours/days", "loop X", "babysit Y", "be proactive about Y every N", or invokes `/loop` directly. Parses `[interval] <prompt>`, picks a clean cron expression, registers the job via the `cron` tool, and executes the prompt once immediately so the user sees activity without waiting for the first cron tick.