Locally built sing-box v1.13.21 carries no version stamp and reports
'unknown', leaving the panel core-version column empty. The agent probe
now falls back to /etc/nexusgate/sing-box/version when the binary
reports no version; agent-singbox.sh writes that record (version
without the v prefix) on every successful install path, including the
already-installed early exit. Agent version stays 0.6.8.
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