Add Screen Recorder mutations to the Workspace assistant

The Workspace assistant now supports starting, appending, finalizing, renaming, sharing, editing, and deleting exact account-owned Screen Recordings, plus creating, updating, and deleting exact image overlays. Existing plan, chunk, storage, timing, geometry, expiry, and cleanup rules remain authoritative. Sharing, public exposure, edit and overlay changes, and deletion require explicit confirmation in the current message. Results contain safe metadata and server-issued links only; video bytes, private paths, share tokens, credentials, and transient signaling are not returned.