Conform to tower's current specs: attachment claims, displacement, say attachments

[shellicar]

Summary

• Attachment claims publish on the conversation's own tree (conv.v2.{id}.attachment.attached/moved/detached), carrying the world/instanceId pair, cwd, tip, and pulse interval.
• A directory change publishes moved under the open claim; nothing publishes on the old agent.v1 attachment subjects.
• A session displaced by another instance stops serving, publishes detached, and tells the user.
• A wire say can carry object-store attachments (images, PDFs): resolved from the bucket the block names, inlined for the model, committed as the reference block; an unresolvable fresh attachment rejects the say attachment_unavailable.
• Change events carry the publishing instance id as envelope provenance, and a tool_result message no longer fabricates a sender.
• Schema and fixture copies are byte-current with tower's, and the conformance suites cover the new shapes.