Remove interactive ISO storage detection/selection and associated ISO_PATH logic. Default to a fixed cache directory (/var/lib/vz/template/iso) via CACHE_DIR and update CACHE_FILE and mkdir usage accordingly. Also update VM configuration to reference the local ISO store (local:iso/<file>) instead of the previously selectable storage. This simplifies ISO handling by avoiding pvesm/whiptail prompts; note this hardcodes the ISO location and removes support for alternate ISO storages.
Introduce an interactive script (vm/cachyos-vm.sh) to create and configure a CachyOS VM on Proxmox. The script validates Proxmox environment and architecture, offers default and advanced settings via whiptail, detects storage/ISO locations, downloads the CachyOS desktop ISO, allocates EFI and main disks, creates and configures the VM (network, CPU, memory, disks, boot order, OVMF), sets a rich VM description, and optionally starts the VM. Also add an ASCII header file (vm/headers/cachyos-vm) used by the script.
