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>

config/samples/providers-config.yaml

@@ -30,8 +30,9 @@ providers:
       knownHostsFile: /etc/proxy-operator/kvm/known_hosts
       # insecureIgnoreHostKey: true   # skip host-key checks (not recommended)
       # diskSizeGb: 10                # per-VM overlay; cloud-init grows the root fs into it
-      # vendorData: |                 # extra vendor-data; the qemu-guest-agent bootstrap is merged
-      #   #cloud-config               # into each Proxy's cloud-config, not configured here
+      # installGuestAgent: false      # skip the vendor-data script that installs qemu-guest-agent
+      # vendorData: |                 # extra vendor-data, carried beside the agent script as a
+      #   #cloud-config               # MIME multipart. The Proxy's own cloud-init is never rewritten.
       #   ...
       images:
         # placement.image names a key here (or is an https:// URL directly).