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>

cmd/felhom-agent/main.go

@@ -44,7 +44,7 @@ import (
 
 // version is the agent version. Overridable at build time with
 // -ldflags "-X main.version=<v>"; defaults to the in-repo CHANGELOG version.
-var version = "0.36.3"
+var version = "0.36.4"
 
 // runGuestHook is the PVE pre-start hook body (`felhom-agent guest-hook <vmid> <phase>`). On the
 // pre-start phase it creates placeholder dirs for any absent bind-mount source so the guest always boots