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/platforms/lxc.nix

@@ -14,7 +14,7 @@ let
   # Without this, config.system.build.tarball's built-in system just
   # generates a fresh host key at first boot like any other host would --
   # but sops-nix derives its decryption key from *this* file, and
-  # .sops.yaml only trusts whatever key scripts/sync-host-keys.sh already
+  # .sops.yaml only trusts whatever key scripts/secrets/sync-host-keys.sh already
   # registered for this exact target name. A freshly-generated key can
   # never match that, so every secret (including this host's own login)
   # permanently fails to decrypt. Confirmed live: sops-install-secrets
@@ -26,7 +26,7 @@ let
   hostKeysDir = /. + hostKeysDirStr;
 
   # flakeTarget ("${platform}-${buildType}") comes in via specialArgs from
-  # flake.nix's mkTarget -- exactly the name scripts/sync-host-keys.sh
+  # flake.nix's mkTarget -- exactly the name scripts/secrets/sync-host-keys.sh
   # registers keys under. Deliberately not read back from
   # config.environment.etc."flake-target" (which is set to the same value)
   # -- this module also *contributes* to environment.etc below, and a