feat: improve agent install release layout support

- fix bare-metal Agent install config and executor path handling
- support release package layout in deploy/install.sh and release workflow
- add regression tests for Agent execution and deploy install script behavior

deploy/install.sh

@@ -1,17 +1,25 @@
 #!/bin/sh
 set -eu
 
-PROJECT_ROOT=$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)
+SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+PROJECT_ROOT=$(CDPATH= cd -- "$SCRIPT_DIR/.." && pwd)
 PREFIX="${PREFIX:-/opt/backupx}"
 ETC_DIR="${ETC_DIR:-/etc/backupx}"
 SERVICE_NAME="backupx"
 APP_USER="backupx"
 APP_GROUP="backupx"
-BIN_SOURCE="${BIN_SOURCE:-$PROJECT_ROOT/server/backupx}"
-WEB_SOURCE="${WEB_SOURCE:-$PROJECT_ROOT/web/dist}"
-CONFIG_TEMPLATE="${CONFIG_TEMPLATE:-$PROJECT_ROOT/server/config.example.yaml}"
+if [ -f "$SCRIPT_DIR/backupx" ] && [ -d "$SCRIPT_DIR/web" ]; then
+    BIN_SOURCE="${BIN_SOURCE:-$SCRIPT_DIR/backupx}"
+    WEB_SOURCE="${WEB_SOURCE:-$SCRIPT_DIR/web}"
+    CONFIG_TEMPLATE="${CONFIG_TEMPLATE:-$SCRIPT_DIR/config.example.yaml}"
+    NGINX_SOURCE="${NGINX_SOURCE:-$SCRIPT_DIR/nginx.conf}"
+else
+    BIN_SOURCE="${BIN_SOURCE:-$PROJECT_ROOT/server/backupx}"
+    WEB_SOURCE="${WEB_SOURCE:-$PROJECT_ROOT/web/dist}"
+    CONFIG_TEMPLATE="${CONFIG_TEMPLATE:-$PROJECT_ROOT/server/config.example.yaml}"
+    NGINX_SOURCE="${NGINX_SOURCE:-$PROJECT_ROOT/deploy/nginx.conf}"
+fi
 SERVICE_SOURCE="${SERVICE_SOURCE:-$PROJECT_ROOT/deploy/backupx.service}"
-NGINX_SOURCE="${NGINX_SOURCE:-$PROJECT_ROOT/deploy/nginx.conf}"
 
 if [ "$(id -u)" -ne 0 ]; then
     echo "请使用 root 或 sudo 执行安装脚本。" >&2
@@ -20,13 +28,20 @@ fi
 
 if [ ! -f "$BIN_SOURCE" ]; then
     echo "未找到后端二进制：$BIN_SOURCE" >&2
-    echo "请先执行：cd \"$PROJECT_ROOT/server\" && go build -o backupx ./cmd/backupx" >&2
+    echo "源码树安装请先执行：cd \"$PROJECT_ROOT/server\" && go build -o backupx ./cmd/backupx" >&2
+    echo "发布包安装请确认当前目录包含 ./backupx、./web 和 ./install.sh。" >&2
     exit 1
 fi
 
 if [ ! -d "$WEB_SOURCE" ]; then
     echo "未找到前端构建产物：$WEB_SOURCE" >&2
-    echo "请先执行：cd \"$PROJECT_ROOT/web\" && npm run build" >&2
+    echo "源码树安装请先执行：cd \"$PROJECT_ROOT/web\" && npm run build" >&2
+    echo "发布包安装请确认当前目录包含 ./web。" >&2
+    exit 1
+fi
+
+if [ ! -f "$CONFIG_TEMPLATE" ]; then
+    echo "未找到配置模板：$CONFIG_TEMPLATE" >&2
     exit 1
 fi
 
@@ -47,11 +62,34 @@ if [ ! -f "$ETC_DIR/config.yaml" ]; then
     install -m 0640 "$CONFIG_TEMPLATE" "$ETC_DIR/config.yaml"
 fi
 
-install -m 0644 "$SERVICE_SOURCE" "/etc/systemd/system/$SERVICE_NAME.service"
+if [ -f "$SERVICE_SOURCE" ]; then
+    install -m 0644 "$SERVICE_SOURCE" "/etc/systemd/system/$SERVICE_NAME.service"
+else
+    cat > "/etc/systemd/system/$SERVICE_NAME.service" <<UNIT
+[Unit]
+Description=BackupX API Service
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=$APP_USER
+Group=$APP_GROUP
+WorkingDirectory=$PREFIX
+ExecStart=$PREFIX/bin/backupx -config $ETC_DIR/config.yaml
+Restart=on-failure
+RestartSec=5
+NoNewPrivileges=true
+LimitNOFILE=65535
+
+[Install]
+WantedBy=multi-user.target
+UNIT
+fi
 systemctl daemon-reload
 systemctl enable --now "$SERVICE_NAME"
 
-if [ -d "/etc/nginx/conf.d" ]; then
+if [ -d "/etc/nginx/conf.d" ] && [ -f "$NGINX_SOURCE" ]; then
     install -m 0644 "$NGINX_SOURCE" "/etc/nginx/conf.d/$SERVICE_NAME.conf"
     if command -v nginx >/dev/null 2>&1; then
         nginx -t