fix(agent): fall back to install record for sing-box core version (item 12)

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.

agent-go/main.go

@@ -35,6 +35,7 @@ type config struct {
 	Root            string // NG_CONFIG_DIR
 	ResourceDir     string // Root + "/resources"
 	SingBoxConfig   string
+	SingBoxVersionFile string
 	SingBoxLog      string
 	SingBoxCursor   string
 	KeyFile         string
@@ -74,6 +75,7 @@ func loadConfig() config {
 		Root:            root,
 		ResourceDir:     root + "/resources",
 		SingBoxConfig:   envOr("NG_SINGBOX_CONFIG", "/etc/nexusgate/sing-box/config.json"),
+		SingBoxVersionFile: envOr("NG_SINGBOX_VERSION_FILE", "/etc/nexusgate/sing-box/version"),
 		SingBoxLog:      envOr("NG_SINGBOX_LOG", "/var/log/nexusgate/sing-box-access.log"),
 		SingBoxCursor:   envOr("NG_SINGBOX_CURSOR", "/etc/nexusgate/sing-box-cursor.json"),
 		KeyFile:         envOr("NG_KEY_FILE", "/etc/nexusgate/keys.json"),