Annotate messages with descriptions of what they're used for

Things like:

• response: Does this message send some kind of response.
• direct: Is this message sent to the user directly?
• channels: Which channels, if any, will this message be sent through?
• side_effects: Other messages (listed here) may be sent as a side-effect of this message. (For instance, the message to update the agenda sends the response "updated" but has the side-effect that the updated agenda item is pushed through the meeting channel)
• Validate the message references through testing, so we know that each referenced message actually exist. (And that responses are outgoing messages for instance)
• Option to cause exceptions if a message is sent in a way that's not declared.
• Create a browsable API(?) :)

Start by locating the message definitions and the tests that reference them. Review how responses, direct messages, channels, and side effects are currently represented. Done would require an agreed annotation design, validation of referenced messages, enforcement behavior, and a decision on whether a browsable API is included.