Archived Agent Host sessions should be read-only in editor chat

Archived Agent Host sessions are read-only in the Agents window, but editor-window chat surfaces should provide the same behavior.

When an archived session is opened in the editor:

- show an "Archived sessions are read-only" banner
- hide the composer and preserve keyboard focus
- prevent send, retry, fork, follow-up, checkpoint restore, and other mutating actions
- react when archive or chat interactivity state changes

Implemented by [#326455](https://github.com/microsoft/vscode/pull/326455).

(Written by Copilot)