feat: agentctl reload — subcomando de hot-reload via SIGHUP

Nuevo subcomando: agentctl reload [agent-id]

- Sin argumento: elimina run/reload.txt y envía SIGHUP → todos los
  agentes se recargan.
- Con agent-id: escribe run/reload.txt con el ID y envía SIGHUP → solo
  ese agente se recarga.

Si el launcher no está corriendo, muestra error claro.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/agentctl/main.go

@@ -14,6 +14,7 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"syscall"
 
 	"github.com/spf13/cobra"
 
@@ -47,6 +48,7 @@ func main() {
 		listCmd(mgr),
 		startCmd(mgr, &binPath),
 		stopCmd(mgr),
+		reloadCmd(mgr),
 		removeCmd(mgr),
 		avatarCmd(),
 		displaynameCmd(),
@@ -171,6 +173,48 @@ func stopCmd(mgr *process.Manager) *cobra.Command {
 	}
 }
 
+// ── reload ────────────────────────────────────────────────────────────────
+
+func reloadCmd(mgr *process.Manager) *cobra.Command {
+	return &cobra.Command{
+		Use:   "reload [agent-id]",
+		Short: "Hot-reload an agent (or all agents) without stopping the launcher",
+		Long: `Sends SIGHUP to the running launcher, which triggers a hot-reload.
+If an agent-id is given, only that agent is reloaded.
+If no agent-id is given, all agents are reloaded.
+
+The launcher must be running. Use 'agentctl start' first if needed.`,
+		Args: cobra.MaximumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			pid := mgr.UnifiedPID()
+			if pid <= 0 {
+				return fmt.Errorf("launcher is not running")
+			}
+
+			target := ""
+			if len(args) == 1 {
+				target = args[0]
+			}
+
+			if target != "" {
+				if err := os.WriteFile("run/reload.txt", []byte(target), 0o644); err != nil {
+					return fmt.Errorf("write reload target: %w", err)
+				}
+				fmt.Printf("reload %-20s  sending SIGHUP to PID %d\n", target, pid)
+			} else {
+				// Remove any stale reload.txt so SIGHUP reloads all agents.
+				_ = os.Remove("run/reload.txt")
+				fmt.Printf("reload %-20s  sending SIGHUP to PID %d\n", "(all)", pid)
+			}
+
+			if err := syscall.Kill(pid, syscall.SIGHUP); err != nil {
+				return fmt.Errorf("kill -HUP %d: %w", pid, err)
+			}
+			return nil
+		},
+	}
+}
+
 // ── remove ────────────────────────────────────────────────────────────────
 
 func removeCmd(mgr *process.Manager) *cobra.Command {