Consolidate minimal-buildtype hosts onto a single nix-minimal identity

linode-minimal, proxmox-minimal, and lxc-minimal now all share
hosts/nix-minimal/host.nix instead of three separate per-platform
host files with different hostnames — every minimal-buildtype host is
named nix-minimal regardless of which platform it runs on.

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

hosts/nix-minimal/host.nix

@@ -0,0 +1,12 @@
+{ ... }:
+
+{
+  # Preserves the hostname of the existing, already-deployed machine
+  # (previously the flat `nix-minimal` target) — the flake attribute name
+  # changed, the real machine's hostname did not.
+  networking.hostName = "nix-minimal";
+
+  # Preserved from the pre-refactor `nix-minimal` target — stateVersion must
+  # never be bumped on an already-installed machine.
+  system.stateVersion = "25.05";
+}