Cross-session messaging gains hook interception and a new plugin API

What

• SendMessage now routes through a hook pipeline that lets a plugin hook rewrite the recipient or the message text before delivery. If the recipient changes, permissions are re-checked. If a hook tries to readdress a send but no ask handler is present to approve it, or the readdress is denied, the send is dropped with an explicit refusal.
• A new plugin-facing API was added for cross-session/agent messaging: $.session.messages reads another agent's or session's transcript, $.session.authorize authorizes a first-party credential for a session, and $.session.send sends a message to another live session or agent (built on the SendMessage tool), including resolving session IDs and reporting delivery-failure reasons.

Why

Together these give plugins more control and visibility over cross-session communication: they can intercept, redirect, or block messages between sessions, and they gain a proper API surface to read another session's history and send it messages programmatically, rather than only through the built-in tool.