Expose the existing CronJob.sessionKey field through the CLI so users
can target cron jobs at specific named sessions without needing an
external shell script + system crontab workaround.
The backend already fully supports sessionKey on cron jobs - this
change wires it to the CLI surface with --session-key on cron add,
and --session-key / --clear-session-key on cron edit.
Closes#27158
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>