Direct clients cannot restore an ActiveSession with session/load or session/resume

[OldKrab]

Problem

SDK 2 made ConnectionTo::attach_session private, while SessionBuilder only handles session/new. A direct ConnectionTo<Agent> therefore cannot turn session/load or session/resume into an ActiveSession.

PR #322 handles proxy forwarding and requires an incoming client Responder, so it does not cover this case.

Required behavior

A stable-v1 direct-client operation should:

• accept either restore request and return an ActiveSession plus its exact response;
• install routing before publishing session/load, preserving replay notifications sent before the response;
• remove routing on failure or cancellation; and
• work without proxy or unstable MCP features.

The interface shape is open for discussion.