Conversation history refused as not registered #115

Summary

Opening a conversation's history is refused for every user, and so is sending a message through the Milo API. Milo now authorizes subresources separately (milo-os/milo#456, milo-os/openfga-provider#139, enabled in datum-cloud/infra#6664 and datum-cloud/infra#6667), and the assistant never registered permissions for either one, so each request fails as "not registered". A read-only probe in production confirmed the history case.

No customer sees this in production yet because the chat dock runs only in staging. It must be fixed before the chat dock is promoted to production. #114 adopted the new model for capability bindings only, and milo-os/telemetry#193 fixed the same gap for logs in telemetry v0.4.18.

Acceptance criteria

• A user who can read conversations can open a conversation's history.
• A user who can start conversations can send a message through the Milo API.
• A user who can only read conversations is refused when sending a message.
• A conversation subresource added later without a permission fails the build.