Cron — safe crontab manager

This skill schedules a command in the user's crontab the safe way: read what is already there, append one line, check the syntax, then install the whole file and verify the old jobs survived.

The non-negotiable part is the backup. crontab <file> replaces the entire user crontab with that file, so the skill always saves the current table first (crontab -l > "$HOME/crontab.backup") and builds the new table from that copy. If the existing table cannot be read, it stops rather than installing over it. crontab -r — which deletes every job — is never run.

It covers the line format (five time fields plus command, with the day-of-month/day-of-week either-match rule), Vixie cron shortcuts like @daily and @reboot, and the environment traps: cron's short PATH and /bin/sh shell mean absolute paths everywhere, output is redirected to a log file you create first, and a literal % in a command must be escaped as \%.

Requirements: the crontab command (Debian/Ubuntu ship it in the cron package, and the daemon must be running; macOS includes it, though launchd is the native scheduler there). On Debian/Ubuntu, crontab -n syntax-checks a file without installing it.