Reorganize scripts/ into secrets/, proxmox/, and lib/ subfolders

scripts/ had grown to 10 top-level scripts covering three distinct
concerns (sops/age + SSH host-key management, Proxmox deployment, and
repo-wide bootstrap/CI) with no grouping. Move the key-management scripts
(backup-admin-key.sh, rotate-admin-key.sh, prepare-host-key.sh,
sync-host-keys.sh) into scripts/secrets/, and the Proxmox scripts
(create-proxmox-resource.sh, configure-nix-cache-client.sh) into
scripts/proxmox/; leave env.sh, codex-setup.sh, codex-maintenance.sh, and
bump-nixpkgs-release.sh at the top level (frequently hand-typed or pure
shared config) and scripts/lib/ as-is.

Updates every cross-reference: each moved script's repo_root computation
(now one directory deeper), shellcheck source= directives, inter-script
paths (create-proxmox-resource.sh's call into sync-host-keys.sh and its
remote bootstrap of configure-nix-cache-client.sh on the Proxmox node),
and every doc/module mention (CLAUDE.md's Scripts section reorganized to
match, README.md, docs/auto-installer.md, docs/proxmox-images.md,
modules/installer/common.nix, modules/platforms/lxc.nix). CI workflows
need no change -- they only invoke codex-maintenance.sh, which didn't
move. Verified via bash -n, shellcheck (no new warnings beyond the
pre-existing SC1091/SC2029/SC2095 baseline), and live dry-runs of
sync-host-keys.sh --all and create-proxmox-resource.sh --list from their
new paths.

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

modules/installer/common.nix

@@ -130,7 +130,7 @@
         # at *activation* time, which runs before systemd would otherwise
         # generate one on first boot. Without pre-seeding it here, secrets
         # (including the login password) fail to decrypt on first boot.
-        # Generate the key with scripts/prepare-host-key.sh first.
+        # Generate the key with scripts/secrets/prepare-host-key.sh first.
         #
         # Two places a key can come from, checked in order:
         #   /etc/host-keys  — baked into this image at build time (see
@@ -150,7 +150,7 @@
         else
             echo "WARNING: no SSH host key found for ''${choice} (checked /etc/host-keys and /root/host-keys)"
             echo "sops-nix secrets (including the login password) will NOT decrypt on first boot."
-            echo "Run scripts/prepare-host-key.sh for host ''${choice} on your admin workstation first,"
+            echo "Run scripts/secrets/prepare-host-key.sh for host ''${choice} on your admin workstation first,"
             echo "then either rebuild this image with NIXOS_HOST_KEYS_DIR set, or scp the result to"
             echo "/root/host-keys/ on this machine."
             read -rp "Continue without a pre-seeded key anyway? (y/N): " skip_key