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>

README.md

@@ -34,7 +34,7 @@ anywhere in this repo — that's live infrastructure state, not something a
 committed file can keep accurate, and it changes independently of the code.
 Check the Proxmox node itself, or `/etc/flake-target` on a running host (see
 below), if you need to know what's really out there right now.
-`scripts/create-proxmox-resource.sh`'s duplicate-host guard works the same
+`scripts/proxmox/create-proxmox-resource.sh`'s duplicate-host guard works the same
 way: it checks the Proxmox node directly rather than any file here.
 
 Each buildtype's `hosts/<name>/host.nix` carries the per-machine identity
@@ -114,7 +114,7 @@ Three different paths depending on target, none of them involving a manual
   disk image and attached to a new VM with no install step — see
   `docs/proxmox-images.md`.
 
-`scripts/create-proxmox-resource.sh --type lxc|vm --host <name>` automates
+`scripts/proxmox/create-proxmox-resource.sh --type lxc|vm --host <name>` automates
 either of the last two end to end (host-key registration, building the
 image directly on the Proxmox node itself, `pct create`/`qm create`), with
 `--dry-run` and a guard against duplicating an already-deployed host's