SendMessage gains a mode limited to the session's own agents

What

SendMessage is the tool Claude uses to send messages to other agents. A new mode, controlled by a flag, limits it to agents that belong to the current session.

• A new check on the target can refuse a send before it happens. The refusal is labelled own_agents_only. The exact condition for refusing was not determined.
• In this mode, the tool's description lists the recipient as an agent of this session by its name and adds "No one outside this session can be reached." The line saying teammate messages are delivered automatically is left out, and so is the older section on protocol responses. The old text always described a "Teammate by name".
• In this mode, the error for a message written as a teammate protocol frame is shorter: "message text must not be a teammate protocol frame (permission/mode/plan/shutdown JSON): send plain text".
• A queued message from a peer is now marked sendMessageToolAbsent when the flag is set, or when a second condition holds and the sender has no senderTaskId.

The standard SendMessage description captured under this release is 4,259 characters, the same as before. The new wording appears only when the mode is on.

Why

In this mode Claude is told, and enforced, that it can only message agents in its own session. Messages to anyone else are refused rather than sent.