Add KVM provider transport: SSH-forwarded libvirt connections and error classification

Per-host connection pool over SSH with the libvirt unix socket forwarded
through the session, lazy dial and redial after disconnect, context
cancellation for go-libvirt's context-less RPCs, handshake deadline and
SSH keepalives. Error classification onto the provider taxonomy. Tested
with a fake dialer and an in-process SSH server that forwards unix sockets.

Step 3 of docs/plans/2026-09-04-1404-kvm-provider.md.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

go.mod

@@ -4,11 +4,13 @@ go 1.26.0
 
 require (
 	cloud.google.com/go/compute v1.65.0
+	github.com/digitalocean/go-libvirt v0.0.0-20260814190004-1a83157e1858
 	github.com/go-logr/logr v1.4.3
 	github.com/kdomanski/iso9660 v0.4.0
 	github.com/onsi/ginkgo/v2 v2.27.4
 	github.com/onsi/gomega v1.39.0
 	github.com/prometheus/client_golang v1.23.2
+	golang.org/x/crypto v0.56.0
 	google.golang.org/api v0.292.0
 	google.golang.org/protobuf v1.36.12-0.20260120151049-f2248ac996af
 	k8s.io/api v0.36.0
@@ -79,7 +81,6 @@ require (
 	go.uber.org/zap v1.27.1 // indirect
 	go.yaml.in/yaml/v2 v2.4.3 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/crypto v0.56.0 // indirect
 	golang.org/x/exp v0.0.0-20251219203646-944ab1f22d93 // indirect
 	golang.org/x/mod v0.38.0 // indirect
 	golang.org/x/net v0.57.0 // indirect