Reactions

Shared reaction semantics across channels:

- ''emoji'' is required when adding a reaction.

- ''emoji=""'' removes the bot's reaction(s) when supported.

- ''remove: true'' removes the specified emoji when supported (requires ''emoji'').

Channel notes:

- ''Discord/Slack'': empty ''emoji'' removes all of the bot's reactions on the message; ''remove: true'' removes just that emoji.

- ''Google Chat'': empty ''emoji'' removes the app's reactions on the message; ''remove: true'' removes just that emoji.

- ''Telegram'': empty ''emoji'' removes the bot's reactions; ''remove: true'' also removes reactions but still requires a non-empty ''emoji'' for tool validation.

- ''WhatsApp'': empty ''emoji'' removes the bot reaction; ''remove: true'' maps to empty emoji (still requires ''emoji'').

- ''Signal'': inbound reaction notifications emit system events when ''channels.signal.reactionNotifications'' is enabled.