Add Proxmox LXC creator script

[modem7]

Adds Bash/Proxmox Scripts/LXC Template/create-lxc.sh, the container counterpart of create-ubuntu-cloud-template.sh.

What it does

• Builds an Ubuntu or Debian container from the Proxmox pveam templates.
• Leaves it running, or cleans it and converts it to a template with --template.
• Same interface as the VM script: prompts, --config, --unattended, --ctid / --auto-ctid, --force-overwrite (templates only).
• Answers can be saved as a profile at the summary.
• Optional features, all off by default, asked in interactive runs or set in a profile: Ansible and admin users (SSH key only), zsh and dotfiles, Postfix relay, unattended-upgrades, snapd removal, IPv6 off, pip system-wide, sudo coredump fix, keyctl.
• Hooks: UPLOAD_FILES and PROVISION_SCRIPTS.
• Clones of a template get their own SSH host keys and mail sender name on first boot (ct-identity.service).

Design notes are in docs/superpowers/specs/2026-10-11-lxc-create-script-design.md.

Testing

• bash -n and shellcheck: clean apart from SC2015 info notes.
• The generated in-container script was run end to end (provision then seal) in debian:13 and ubuntu:24.04 Docker containers with every feature on.
• Container mode run on Proxmox VE 9.2 (Ubuntu 26.04, static IP, VLAN, admin user): built and left running.
• Template mode (--template) has not been run on a Proxmox host yet. The interactive feature prompts were tested with scripted answers only.