Merge the qemu-guest-agent bootstrap into each Proxy's cloud-config instead of vendor-data

In the kind cluster two KVM Proxies with real cloud-configs booted fine
but never left Provisioning: the guest agent was not installed, so the
provider could not learn the IP. cloud-init merges vendor-data under
user-data with its default mergers, which keep the user-data's value
for any key both define and never append lists, so a cloud-config with
its own `packages:` silently dropped the vendor-data's packages and
runcmd. The integration test passed only because its user-data was
empty.

withGuestAgent now appends qemu-guest-agent to `packages` and the
enable command to `runcmd` of the Proxy's own cloud-config (skipping
entries already present) and re-renders it. Vendor-data is no longer
defaulted; `vendorData` is plain extra data. User-data that is not a
cloud-config document (script, #include, MIME) still gets the bootstrap
as vendor-data. A cloud-config that is not a YAML mapping, or whose
packages/runcmd is not a list, fails the Proxy as ErrPermanent.

Validated in-cluster: both Proxies Ready and healthy in two minutes, 204
through squid from the LAN. Docs, sample comment, CHANGELOG and the
plan execution log (Run 7) updated.

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

internal/provider/kvm/kvm.go

@@ -42,9 +42,6 @@ func withDefaults(cfg provider.KVMConfig) provider.KVMConfig {
 	if cfg.DiskSizeGB == 0 {
 		cfg.DiskSizeGB = defaultDiskSizeGB
 	}
-	if cfg.VendorData == "" {
-		cfg.VendorData = defaultVendorData
-	}
 	images := make(map[string]provider.KVMImage, len(cfg.Images))
 	for k, img := range cfg.Images {
 		if img.Format == "" {
@@ -249,7 +246,11 @@ func (p *Provider) writeSeed(log logr.Logger, api libvirtAPI, pool libvirt.Stora
 	if err := deleteIfExists(api, pool, seedName); err != nil {
 		return libvirt.StorageVol{}, err
 	}
-	iso, err := buildSeedISO(name, userData, p.cfg.VendorData)
+	userData, vendorData, err := withGuestAgent(userData, p.cfg.VendorData)
+	if err != nil {
+		return libvirt.StorageVol{}, fmt.Errorf("%w: %v", provider.ErrPermanent, err)
+	}
+	iso, err := buildSeedISO(name, userData, vendorData)
 	if err != nil {
 		return libvirt.StorageVol{}, fmt.Errorf("%w: %v", provider.ErrPermanent, err)
 	}