v0.41.0: provision back-half sets onboot:1 so customer guests auto-start after host reboot (F3)

The golden bakes --onboot 0 (template safety) and the back-half never overrode it, so every
provisioned customer guest was onboot:0 -> after a host reboot/power-cut the customer's whole
home-server stayed stopped until a manual pct start. Add a fatal 'pct set <vmid> -onboot 1' step
to BackHalf.Provision (right after the config-mount attach), mirroring the existing pct set ops.
No startup/boot-order: the v0.75 mountpoint-gate covers the drive-bind race at boot.

Golden build-golden.sh unchanged (templates must not auto-start). Unit-tested
(TestProvision_SetsOnbootOne + red-proof). RUNBOOK note added.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FpBYrZCt9sFDqLgbG5GRGD

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 = "0.40.0"
+var version = "0.41.0"
 
 // 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