Install the guest agent from a vendor-data script instead of editing the user's cloud-config

Merging the bootstrap into each Proxy's cloud-config was the wrong
mechanism twice over. Detection only matched a literal #cloud-config
first line, so a jinja template or a MIME archive still fell back to
vendor-data, where cloud-init's default mergers drop it — the exact
failure the merge existed to prevent. And the YAML round-trip re-typed
unquoted scalars: version: 1.10 reached the guest as 1.1, comments were
dropped and keys reordered.

Neither is worth patching, because vendor-data delivered as a shell
script bypasses the merger entirely: cloud-init hands vendor-data to its
own handler chain, writes scripts to a separate directory and runs them
from cc_scripts_vendor, after the user's packages and before the user's
runcmd. The merge could not have been rescued anyway — merge_how in
user-data governs only the first source, and merge_how in vendor-data is
discarded before the document is stored.

The Proxy's cloud-init now reaches the seed byte for byte in any format.
installGuestAgent (default true) turns the script off; a configured
vendorData travels beside it as a MIME multipart, declared text/plain so
cloud-init re-sniffs it and this code never classifies user content.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/provider/kvm/kvm.go

@@ -250,7 +250,7 @@ func (p *Provider) writeSeed(log logr.Logger, api libvirtAPI, pool libvirt.Stora
 	if err := deleteIfExists(api, pool, seedName); err != nil {
 		return libvirt.StorageVol{}, err
 	}
-	userData, vendorData, err := withGuestAgent(userData, p.cfg.VendorData)
+	vendorData, err := buildVendorData(p.cfg.VendorData, p.cfg.InstallGuestAgent == nil || *p.cfg.InstallGuestAgent)
 	if err != nil {
 		return libvirt.StorageVol{}, fmt.Errorf("%w: %v", provider.ErrPermanent, err)
 	}