New session.send hook event and plugin API for sending messages into a session

What

• A new hookable event session.send was added to the plugin event system, alongside the existing session.receive, session.detach, and session.end. It has its own schema (checkArgument, restoreArgument, check), is included in the events that can trigger the "hook ... is holding" warning if answered late, and runs through the same delivery pipeline as tool calls, tracking isDelivered/reason with a "the message was not delivered" failure message.
• The plugin/SDK host API now exposes a $.session.send operation that lets a plugin send text to a named target: a session ID, an agent ID, or an address object ({sessionId} or {agentId}). It requires non-empty to and text values.

Why This lets plugins and SDK integrations both react to and initiate messages being sent into a session, rather than only observing incoming ones.