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>

scripts/env.sh

@@ -3,10 +3,10 @@
 # second copy of these values in every script:
 #   source "$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/env.sh"
 # Every variable can still be overridden per-invocation via the
-# environment (e.g. PROXMOX_STORAGE=tank-nvme ./scripts/create-proxmox-resource.sh ...)
+# environment (e.g. PROXMOX_STORAGE=tank-nvme ./scripts/proxmox/create-proxmox-resource.sh ...)
 # since each one only sets a default if unset.
 
-# SSH-reachable Proxmox node that scripts/create-proxmox-resource.sh runs
+# SSH-reachable Proxmox node that scripts/proxmox/create-proxmox-resource.sh runs
 # pct/qm on. Matches the Proxmox web UI hostname already used in
 # hosts/nixos/home.nix's desktop shortcuts (pve.<homeDomain> from
 # variables.nix) -- change this if that's not actually reachable over SSH,
@@ -15,7 +15,7 @@
 : "${PROXMOX_SSH_USER:=root}"
 
 # Where this flake repo lives on the Proxmox node itself.
-# scripts/create-proxmox-resource.sh builds images directly on the node
+# scripts/proxmox/create-proxmox-resource.sh builds images directly on the node
 # instead of transferring them over the network -- it clones the repo here
 # (from this checkout's own `origin` remote) the first time it doesn't
 # find it, installing build tooling via scripts/codex-setup.sh, then