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/config_kvm.go

@@ -23,10 +23,17 @@ type KVMConfig struct {
 	// guest's cloud-init growpart expands the root filesystem into it.
 	DiskSizeGB int64 `json:"diskSizeGb,omitempty"`
 	// VendorData is optional cloud-init vendor-data placed on every seed next
-	// to the Proxy's own user-data. cloud-init drops any top-level key here
-	// that the user-data also defines. The qemu-guest-agent bootstrap does
-	// not live here: the provider merges it into each Proxy's cloud-config.
+	// to the Proxy's own user-data. Note that cloud-init drops any top-level
+	// key of a vendor-data cloud-config that the user-data also defines.
+	// When InstallGuestAgent is on, this document and the agent script are
+	// carried as separate parts of a MIME multipart vendor-data.
 	VendorData string `json:"vendorData,omitempty"`
+	// InstallGuestAgent adds a vendor-data script that installs and starts
+	// qemu-guest-agent, which is how the provider learns a VM's address
+	// before the host's ARP table has seen traffic from it. Default true.
+	// Turn it off for images that already ship the agent, or that cannot
+	// reach a package mirror; discovery then depends on ARP alone.
+	InstallGuestAgent *bool `json:"installGuestAgent,omitempty"`
 	// Images are named base images; placement.image names a key here (or is
 	// an http(s) URL directly). The provider downloads each image into a
 	// host's storage pool on first use and caches it there.