Enables read-only mode for these channels by adding actions.sendMessage
configuration option. When set to false, blocks outbound messages while
still allowing message reception.
Follows the existing Telegram/WhatsApp pattern:
- Add sendMessage?: boolean to channel ActionConfig types
- Check gate in action handler, throw error if disabled
- Default: true (enabled) when not specified
This brings read-only mode support to 6 out of 7 core channels
(Telegram, WhatsApp, Discord, Slack, Google Chat, Matrix).
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