Explicit communication lets agents choose when to reply

Every agent now has a Communication setting. Direct, the default, keeps today’s behavior: the assistant text is the reply. Explicit treats assistant text as private working notes; the agent talks only by calling send_message and can call no_reply to stay silent.

This fits Slack channels and conversations with several people, where the agent should answer only when it has something to add. The setting is on the agent in the web UI, the API, agent packages, the Framework builder and the CLI, and it replaces Slack’s per-endpoint reply mode. Every sent message is recorded as a conversation.message event.

See Explicit communication and #4447.