chore(secrets): Phase 4 — remove stale sops.yaml anchors and re-encrypt

Remove four stale age key anchors from .sops.yaml that correspond to
non-lxc build-type variants that were never deployed (or are now
superseded by their lxc-* counterparts):

  &docker   → superseded by &lxc-docker (active running host)
  &server   → superseded by &lxc-server (active running host)
  &nix-cache → superseded by &lxc-nix-cache (active running host)
  &nix-minimal → superseded by &lxc-minimal (active running host)

Also remove the secrets/docker.yaml creation_rules block entirely since
that file does not exist.

Re-encrypt secrets/common.yaml, secrets/nix-cache.yaml, and
secrets/server.yaml with sops updatekeys to drop the stale recipients.
The four removed keys can no longer decrypt these files.

Update README.md and CLAUDE.md to clarify that deployed lxc-* hosts
now use clan vars (vars/per-machine/<target>/openssh/) rather than the
gitignored host-keys/ directory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B2EJ4qTsM5KUqhS5c3GAwx

CLAUDE.md

@@ -27,9 +27,12 @@ machines when deployed.
   template for a *real* host — every other host uses sops-nix
   (`hashedPasswordFile`, see "Security Notes" in `README.md`). Flag any *new*
   secret-like string you encounter instead of committing it.
-- `host-keys/` is gitignored — locally-generated *private* SSH host keys for
-  the auto-installer (see `docs/auto-installer.md`). Never commit its
-  contents; if `git status` ever shows it as trackable, something is wrong.
+- `host-keys/` is gitignored — locally-generated *private* SSH host keys
+  for the auto-installer and non-LXC hosts (see `docs/auto-installer.md`).
+  Never commit its contents; if `git status` ever shows it as trackable,
+  something is wrong. Deployed LXC hosts use clan vars
+  (`vars/per-machine/<target>/openssh/`, committed and sops-encrypted)
+  instead — those ARE tracked by git and belong in the repo.
 
 ### Two Proxmox nodes: `pve1.sweet.home` (production) and `pve-test.sweet.home` (sandbox)
 
@@ -203,8 +206,11 @@ instead of copying it.
 - `scripts/secrets/sync-host-keys.sh` — generates/registers SSH host keys
   and their `.sops.yaml`/`secrets/*.yaml` recipients for flake targets,
   idempotently (`--all`, `<target>`, `--remove`, `--regenerate-all-keys`,
-  all with `--dry-run`). The primary tool for provisioning a new host's
-  secrets access — see "Creating a new machine" in `docs/auto-installer.md`.
+  all with `--dry-run`). For lxc-* targets it stores keys as clan vars
+  (`vars/per-machine/<target>/openssh/`, committed and sops-encrypted);
+  for other targets it uses the gitignored `host-keys/` directory. The
+  primary tool for provisioning a new host's secrets access — see
+  "Creating a new machine" in `docs/auto-installer.md`.
 - `scripts/secrets/prepare-host-key.sh` — narrower predecessor: generates a
   key by an arbitrary name without touching `.sops.yaml`. Still useful to
   pre-generate a key before its flake target exists yet, since