Follow-up to #11909: surface the legacy-unit warning where users are most
likely to see it. After a 'hermes update', if a pre-rename hermes.service
is still installed alongside the current hermes-gateway.service, print
the list of legacy units + the 'hermes gateway migrate-legacy' command.
Profile-safe: reuses _find_legacy_hermes_units() which is an explicit
allowlist of hermes.service only — profile units never match.
Platform-gated: only prints on systemd hosts (the rename is Linux-only).
Non-blocking: just prints, never prompts, so gateway-spawned
hermes update --gateway runs aren't affected.
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