Fix stale documentation: outdated counts, missing build type, spec status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

AGENTS.md

@@ -7,7 +7,7 @@ servers and workstation.
 
 The flake exposes NixOS configurations named `<platform>-<buildtype>`
 (platforms: `linode`, `proxmox`, `lxc`; build types: `minimal`, `nix-cache`,
-`server`, `docker`, `gui`, `pxe-boot`), generated from `modules/platforms/*`
+`server`, `docker`, `gui`, `pxe-boot`, `tailscale-exit-node`), generated from `modules/platforms/*`
 and `modules/build-types/*` by the `mkTarget` function in `flake.nix`. Not
 every combination is built — `pxe-boot` has no `linode` variant. See
 `README.md` for the full current target list; treat `flake.nix` as the