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>

docs/proxmox-images.md

@@ -7,7 +7,7 @@ config (`modules/disko/proxmox.nix`) already used to format a real disk on
 install, so there's nothing host-specific to write; it's available for every
 `proxmox-*` target automatically.
 
-`scripts/create-proxmox-resource.sh --type vm --host <name>` automates the
+`scripts/proxmox/create-proxmox-resource.sh --type vm --host <name>` automates the
 whole walkthrough below (and the equivalent LXC one) end to end, including
 host-key handling and building the image directly on the Proxmox node
 itself (no local build, no image transfer) — see its `--help`. The steps
@@ -51,7 +51,7 @@ sudo ./result \
   --build-memory 2048
 ```
 
-Generate the key first with `scripts/sync-host-keys.sh <hostname>`, same
+Generate the key first with `scripts/secrets/sync-host-keys.sh <hostname>`, same
 as any other host — see `docs/auto-installer.md` for the full walkthrough
 (it registers the new key in `.sops.yaml` and re-encrypts the affected
 `secrets/*.yaml` files too, no manual editing needed).