Add tools.cron.command_allowed_remotes so remote channels can be
explicitly allowlisted for command-executing cron jobs. Entries support
channel names, channel:chat_id pairs, and a literal * to allow every
non-empty channel while preserving the default deny posture.
The existing allow_command and command_confirm checks still apply after the
channel gate. Update tests, config examples, and cron documentation for the
new option.
Add GetJob and improved UpdateJob to CronService with proper cloning,
schedule diffing, and next-run recomputation. Expose get/update actions
in the cron tool so agents can inspect and partially update jobs without
losing payloads or needing remove+add cycles. Includes access control
for remote channels and command safety gates.
