Both remove_agent() calls now run unconditionally for maximum partial
cleanup, but any I/O error is returned as HTTP 500 instead of silently
200-ing — so the frontend's !resp.ok path fires and the operator sees a
meaningful error rather than the stale row reappearing unchanged.
Also add a clarifying comment on isStale in permissions.js explaining
that containersState is keyed from nixos-container list (which includes
stopped-but-configured containers), so a temporarily-stopped agent is
not treated as stale — only destroyed/renamed agents are absent.
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