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/auto-installer.md

@@ -74,8 +74,8 @@ booting one:
 First boot runs `boot.postBootCommands` (registers the Nix store DB and
 system profile) — there's no separate activation step to run yourself.
 `scripts/create-proxmox-resource.sh --type lxc --host <name>` automates all
-of this (build, host-key handling, upload, `pct create` with the flags
-above) — see its `--help`.
+of this (host-key handling, building the tarball directly on the Proxmox
+node itself, `pct create` with the flags above) — see its `--help`.
 
 Host keys still need pre-seeding the same way as any other host — the
 sops-nix activation-vs-first-boot race is identical regardless of how the