Consolidate CI on codex-maintenance.sh; clean up dead scripts and docs

Both check-nixos.yml workflows (GitHub + Gitea) now call
scripts/codex-maintenance.sh instead of a hand-rolled eval-only loop,
closing a real gap: CI previously enforced none of the secret grep,
nixpkgs-fmt, or statix checks that codex-maintenance.sh already runs
locally — nothing was stopping that from regressing. One script now
backs both, instead of two copies that can drift from each other.

codex-maintenance.sh itself is extended to cover buildable surface
that wasn't validated anywhere before: packages.x86_64-linux.*, plus
config.system.build.tarball (lxc-* hosts) and
config.system.build.diskoImagesScript (proxmox-*, excluding the
installer's own proxmox-lxc target, which has no disko config).

Also:
- scripts/prepare-host-key.sh: dropped the redundant
  [path-to-nixos-repo] parameter — it always defaults to the repo the
  script itself lives in now, so a second argument never made sense
  after the nix-auto-installer migration.
- Removed prepare.sh (dead pre-disko manual parted/mkfs/mkswap
  partitioning, fully superseded) and
  scripts/create-linode-installer-disk.sh (incomplete draft for an
  abandoned dd-via-rescue-mode approach; Linode hosts already deploy
  fine through the normal auto-installer flow).
- docs/pxe-boot.md: fixed a stale `nixosConfigurations.pxe-boot` eval
  command (pre-refactor flat name, not a real flake attribute
  anymore) and added a cross-reference to docs/auto-installer.md.
- CLAUDE.md/README.md: full documentation pass reconciling this
  session's changes — modules/installer/, modules/pxe-boot/, the
  LXC/Proxmox image-building deployment paths, corrected the
  password-hash/SSH-key locations in the safety-rules section (both
  had drifted to reference files/paths that no longer exist), and
  added session-workflow guidance to prefer targeted host evals over
  full-repo sweeps for incremental changes (explicitly scoped to
  interactive sessions, not CI).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

.github/workflows/check-nixos.yml

@@ -17,13 +17,5 @@ jobs:
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@v19
 
-      - name: Evaluate all NixOS hosts
-        run: |
-          set -euo pipefail
-          hosts="$(nix --extra-experimental-features 'nix-command flakes' eval --json \
-            .#nixosConfigurations --apply builtins.attrNames | jq -r '.[]')"
-          for host in $hosts; do
-            echo "Evaluating ${host}"
-            nix --extra-experimental-features 'nix-command flakes' eval \
-              ".#nixosConfigurations.${host}.config.system.build.toplevel.drvPath" --raw
-          done
+      - name: Run maintenance checks (secrets, fmt, lint, eval)
+        run: bash scripts/codex-maintenance.sh