Signal's groupPolicy: allowlist was checking senders against groupAllowFrom,
but users expected to put group IDs there. This adds proper group-level
allowlisting via channels.signal.groups.<groupId>, matching the pattern
used by Telegram and iMessage.
When a group is explicitly listed in the groups config, it bypasses the
sender-level groupAllowFrom check, allowing all members of that group
to interact with the bot.
- Add SignalGroupConfig type and groups property to SignalAccountConfig
- Add resolveChannelGroupPolicy check before sender-level gating
- Add tests for group-level allowlist functionality
- Update docs to clarify groupAllowFrom vs groups config
