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.

scripts/agent-singbox.sh

@@ -9,7 +9,16 @@ set -Eeuo pipefail
 version='v1.13.21'
 target='/usr/local/bin/nexusgate-sing-box'
 stats_target='/usr/local/bin/nexusgate-sing-box-stats'
+# The locally built sing-box carries no version stamp (`sing-box version`
+# prints "unknown"), so the agent cannot probe the core version from the
+# binary. Record the installed version (without the leading v) for the
+# agent's fallback probe — see agent-go installedSingBoxVersion.
+write_version_record() {
+  install -d -m 0700 /etc/nexusgate/sing-box
+  printf '%s\n' "${version#v}" > /etc/nexusgate/sing-box/version
+}
 if [[ -x "$target" && -x "$stats_target" && "${2:-}" != '--force' ]]; then
+  write_version_record
   printf 'sing-box 已安装：%s（如需重装，运行 ng-agent-singbox install --force）\n' "$target"
   exit 0
 fi
@@ -38,6 +47,7 @@ try_prebuilt() {
       install -m 0755 "$tmp_stats" "${stats_target}.next"
       mv -f "${target}.next" "$target"
       mv -f "${stats_target}.next" "$stats_target"
+      write_version_record
       printf 'sing-box 预编译版安装完成：%s\n' "$target"
       return 0
     fi
@@ -104,6 +114,7 @@ install -m 0755 "$build_dir/sing-box" "$target.next"
 install -m 0755 "$build_dir/stats-query" "$stats_target.next"
 mv -f -- "$target.next" "$target"
 mv -f -- "$stats_target.next" "$stats_target"
+write_version_record
 printf 'sing-box 统计版已安装：%s\n' "$target"
 if [[ -f /etc/nexusgate/sing-box/config.json ]]; then
   if command -v systemctl >/dev/null && [[ -d /run/systemd/system ]]; then