feat(server): agent 热重载 sing-box(SIGHUP)替代全量重启(#3)

sing-box 支持 SIGHUP 热重载(cmd_run.go:进程内 check()验证→close 旧实例→
start 新实例;坏配置保留旧实例继续跑、不断网)。原 agent 用 systemctl restart
(全量重启:断所有连接 + 坏配置让 sing-box 起不来全断)。

- Restarter 接口加 Reload;SystemdRestarter.Reload 取 MainPID(systemctl show
  -p MainPID)发 SIGHUP——agent 与 sing-box 同 pangolin 用户,免 polkit/不改 unit;
  PID 取不到或发信号失败回退 Restart
- SingBox.started:首次/进程未起走 Restart(冷启动),之后配置变更走 Reload
- noopRestarter/fakeRestarter 补 Reload;加 TestWriteAndRestartFirstColdThenReload

价值:切节点/续期/加删用户不整进程重启、不全断流;坏配置不至于打死节点。

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

server/internal/agentd/util.go

@@ -7,6 +7,9 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strconv"
+	"strings"
+	"syscall"
 )
 
 // logf is the agent's structured-ish logger. It deliberately never logs
@@ -48,15 +51,44 @@ type SystemdRestarter struct {
 	Unit string // e.g. "sing-box"
 }
 
-// Restart runs `systemctl restart <unit>`.
+// Restart runs `systemctl restart <unit>` (cold start: ~1-2s data-plane blip).
 func (r SystemdRestarter) Restart(ctx context.Context) error {
-	unit := r.Unit
-	if unit == "" {
-		unit = "sing-box"
-	}
+	unit := r.unit()
 	cmd := exec.CommandContext(ctx, "systemctl", "restart", unit)
 	if out, err := cmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("agentd: restart %s: %w: %s", unit, err, out)
 	}
 	return nil
 }
+
+// Reload hot-reloads sing-box via SIGHUP to its MainPID. sing-box validates the
+// new config first and keeps the OLD instance running on error (no full
+// process restart). agent 与 sing-box 同 pangolin 用户,直接发信号即可(免
+// polkit/不改 unit)。PID 取不到或发信号失败时回退到 Restart(冷启动)。
+func (r SystemdRestarter) Reload(ctx context.Context) error {
+	unit := r.unit()
+	pid, err := singboxMainPID(ctx, unit)
+	if err != nil || pid <= 0 {
+		return r.Restart(ctx)
+	}
+	if err := syscall.Kill(pid, syscall.SIGHUP); err != nil {
+		return r.Restart(ctx)
+	}
+	return nil
+}
+
+func (r SystemdRestarter) unit() string {
+	if r.Unit == "" {
+		return "sing-box"
+	}
+	return r.Unit
+}
+
+// singboxMainPID 读 systemd 记录的主进程 PID(0 表示未在跑)。
+func singboxMainPID(ctx context.Context, unit string) (int, error) {
+	out, err := exec.CommandContext(ctx, "systemctl", "show", "-p", "MainPID", "--value", unit).Output()
+	if err != nil {
+		return 0, err
+	}
+	return strconv.Atoi(strings.TrimSpace(string(out)))
+}