Remote Control sessions can fetch messages from a session inbox

What

Remote Control sessions previously received a feature_disabled failure whenever they tried to fetch a message from a session's "inbox". This is now a real feature:

• A new tool, FetchInboxMessage, reads a single message from a session's inbox by file_id. It's used when a session receives a session-inbox notification, for example a message relayed from a linked Slack/Teams/chat thread or the Claude Code project thread.
• The underlying handler now performs the real work: it re-checks whether the session owner has changed, refreshes the auth token if needed, and fetches the message using the session id, file id, base URL, and access/trusted-device tokens.

The returned message is wrapped with trust markers. Only messages whose outer envelope is marked from="rc_owner" are treated as the user's own request; everything else is explicitly untrusted third-party text that must not be acted on without checking with the user first.

Why

This lets Remote Control sessions actually retrieve inbox messages relayed from other chat surfaces, while keeping a clear boundary between the user's own instructions and third-party text so the assistant doesn't act on untrusted content by mistake.