Build Proxmox images directly on the node instead of transferring them

create-proxmox-resource.sh no longer builds locally and scp's a
multi-gigabyte image over -- it now clones/pulls this repo onto the
Proxmox node itself (bootstrapping build tooling via the existing
codex-setup.sh on first use) and runs the nix build / disko image
script there, staging the result straight into the node's own import
directory. host-keys/ (gitignored) is copied over separately since a
git pull doesn't carry it. --image still uploads an explicit local
file for the case where you don't want a build at all.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/proxmox-images.md

@@ -9,9 +9,11 @@ install, so there's nothing host-specific to write; it's available for every
 
 `scripts/create-proxmox-resource.sh --type vm --host <name>` automates the
 whole walkthrough below (and the equivalent LXC one) end to end, including
-host-key handling and upload — see its `--help`. The steps here are what it
-runs under the hood, useful for doing any of it by hand or understanding
-what it does before you trust it against real infrastructure.
+host-key handling and building the image directly on the Proxmox node
+itself (no local build, no image transfer) — see its `--help`. The steps
+here are what it runs under the hood, useful for doing any of it by hand
+or understanding what it does before you trust it against real
+infrastructure.
 
 ## Building