Hosted macOS agent control: stim stop leaves agent-device bound to the old remote config, and host session dirs pile up #2522

Problem

From the two-Mac e2e of the agent-device macos-app lease (#2477, results in issuecomment-6001026253):

• After stim stop of a hosted macOS app, the client's local agent-device session stays bound to that app's remote config. The next hosted workspace's --remote-config fails with "A different remote connection is already active" until agent-device disconnect is run by hand.
• On the host, ~/.stim/server/agent-device/sessions/stim.<session>_default directories stay behind after every hosted session.

Expected

• stim stop / worktree remove for a hosted macOS app also ends the client's agent-device connection for that remote config (agent-device close + disconnect for that config only), or the guide states the exact commands agents run before stopping. The guide's agent section covers it either way.
• stim-server removes a hosted session's agent-device session directory when the session stops (it owns ~/.stim/server).