Fix stale documentation: outdated counts, missing build type, spec status

Same class of problem as the deployedTargets/README fixes: hand-maintained
prose that drifted from reality and nobody was obligated to update.

- CLAUDE.md: "18 hosts" was a stale hardcoded count (actually 20); reworded
  to not need updating as hosts are added. Also added the missing
  tailscale-exit-node build type to a list that had it everywhere else in
  the file except one bullet.
- AGENTS.md: same missing tailscale-exit-node build type.
- docs/auto-installer.md: the hand-enumerated lxc-* list was missing
  lxc-tailscale-exit-node.
- flake-target-refactor-spec.md: added a "Status: implemented" note so this
  completed historical spec (referenced elsewhere purely for rationale)
  can't be mistaken for an open plan with unresolved Open Questions.
- remove-sensetive-info-refactor.md: the "Definition of done" checklist was
  entirely unchecked despite most of the work being done. Checked off what's
  actually done (sops-nix migration, history scrub just performed, the
  pre-commit gitleaks hook), and left rotation of the GitHub PAT found in
  history explicitly flagged as the one still-open item -- an operator
  action against GitHub, not something this repo can attest to itself.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CLAUDE.md

@@ -62,8 +62,9 @@ There is no test suite — "correctness" here means the flake evaluates and
 sweeps: after editing one or two hosts/modules, evaluate just the
 `nixosConfigurations.<host>` you touched (plus any `config.system.build.tarball`
 /`diskoImagesScript`/package output affected) rather than looping over every
-host — `codex-maintenance.sh` evaluates 18 hosts plus every package/tarball/
-image variant now and is slow to run after each small change. Reserve a full
+host — `codex-maintenance.sh` evaluates every `nixosConfigurations` host plus
+every package/tarball/image variant and is slow to run after each small
+change. Reserve a full
 `codex-maintenance.sh` run for changes that plausibly affect every host
 (`modules/common/*`, `flake.nix`, `variables.nix`) or as a final check before
 committing. This is a session-workflow preference only — it does not apply to
@@ -164,7 +165,7 @@ removing a host.
   `vzdump` backup-archive metadata this doesn't have), no install step —
   see `docs/auto-installer.md`.
 - `modules/build-types/*.nix` — what a system is for:
-  minimal/server/docker/gui/pxe-boot/nix-cache.
+  minimal/server/docker/gui/pxe-boot/nix-cache/tailscale-exit-node.
 - `modules/common/configuration.nix` — base NixOS config imported by every
   host: locale, users, nix settings, git.
 - `modules/common/home.nix` / `hosts/nixos/home.nix` — Home Manager config for