agent v0.36.4: serialize AttachDrive/DetachDrive (no double-bind TOCTOU race)

A GuestBinder mutex prevents a concurrent reconnect + periodic reconcile from
both passing isHostMountpoint and double-binding a stable path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/localapi/guestbind.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log/slog"
 	"strconv"
+	"sync"
 
 	"gitea.dooplex.hu/admin/felhom-agent/internal/proxmox"
 )
@@ -35,6 +36,10 @@ const felhomDataNS = "felhom-data"
 type GuestBinder struct {
 	runner proxmox.Runner
 	logger *slog.Logger
+	// mountMu serializes AttachDrive/DetachDrive so a concurrent reconnect (controller-triggered) and the
+	// agent's periodic reconcile can't both pass the isHostMountpoint check and double-bind the same
+	// stable path (a TOCTOU race — observed live as 2 stacked binds).
+	mountMu sync.Mutex
 }
 
 // NewGuestBinder builds a binder over the given root-CLI runner.