Serve-only cloud clients now detect and report being unlinked

What

• The headless cloud client, when running in "serve-only" mode (meaning this computer is just acting as a tool host for a cloud session rather than driving it), now reacts to being unlinked mid-session: it handles a tool_host_ended message from the server and also periodically rechecks whether it's still linked, ending its connection if not.
• Its session-end reasons now include a distinct tool_host_ended value, separate from existing reasons like disconnected, rejected_options, open_failed, and output_failed, with its own exit message. This new reason only fires for serve-only (control-only) clients.

Why

Without this, a serve-only helper client could keep running even after the cloud session it was serving had ended or unlinked it, instead of shutting down promptly with a clear reason.