Reaction tooling

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.