feat: deploy.sh 支持一键部署与就绪探测

- 新增 up 子命令：build + start + watchdog 一键强制重来
- start 改为轮询 /api/health 就绪才返回，消除启动竞态误报
- start 增加端口占用守卫：非本项目进程占用时明确报错
- watchdog 增加启动宽限(WATCHDOG_STARTUP_GRACE)，避开启动窗口误重启
- watchdog 支持 launchd 模式(WATCHDOG_RESTART_MODE)，用 launchctl
  kickstart 重启以避免与 KeepAlive 抢端口
- 抽取 _spawn_server/_wait_ready/foreign_port_holder 公共函数

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

deploy/install-service.sh

@@ -95,6 +95,8 @@ EOF
     <dict>
         <key>PATH</key>
         <string>$VENV_DIR/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin</string>
+        <key>WATCHDOG_RESTART_MODE</key>
+        <string>launchd</string>
     </dict>
     <key>RunAtLoad</key>
     <true/>