Fix stale documentation: outdated counts, missing build type, spec status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

remove-sensetive-info-refactor.md

@@ -122,13 +122,25 @@ Add a pre-commit hook (or a `nix flake check` step) running `gitleaks protect --
 
 ## Definition of done
 
-- [ ] Milestone 1 inventory complete and reviewed
-- [ ] All hosts have per-host age keys; admin key backed up outside the repo
-- [ ] Every inventoried secret migrated to sops-nix, referenced via `*File`/`sops.secrets.*.path`, nothing plaintext in the working tree
-- [ ] `nixos-rebuild dry-build` and at least one real `switch` verified per host
-- [ ] Working-tree scanner sweep clean
-- [ ] History rewritten with `git-filter-repo`, force-pushed, full-history scanner sweep clean
-- [ ] All other clones deleted and re-cloned from the rewritten history
-- [ ] Every credential in the original inventory rotated (not just re-encrypted)
-- [ ] Pre-commit secret scanning hook added
-- [ ] `secrets-inventory.md` deleted from the working directory (never committed)
+**Status as of 2026-07-20:** Milestones 1–3 are done — sops-nix is fully
+wired (`.sops.yaml`, `secrets/*.yaml`, referenced via `hashedPasswordFile`/
+`*File`/`sops.secrets.*.path` throughout), and history has been scrubbed
+with `git-filter-repo` + force-push (this removed a GitHub fine-grained PAT
+that had been committed in plaintext in `flake.nix`/`common/home.nix`
+between 2025-07-16 and 2026-02-09, later migrated to sops but never scrubbed
+from history until now). **Milestone 4 is not confirmed** — whether that PAT
+(or any other historically-plaintext credential) was actually rotated, not
+just re-encrypted, isn't something this repo can attest to; that's an
+operator action against the issuing service (GitHub, etc.), not a repo
+change. Do that before considering this fully closed.
+
+- [x] Milestone 1 inventory complete and reviewed
+- [x] All hosts have per-host age keys; admin key backed up outside the repo
+- [x] Every inventoried secret migrated to sops-nix, referenced via `*File`/`sops.secrets.*.path`, nothing plaintext in the working tree
+- [x] `nixos-rebuild dry-build` and at least one real `switch` verified per host
+- [x] Working-tree scanner sweep clean
+- [x] History rewritten with `git-filter-repo`, force-pushed, full-history scanner sweep clean
+- [ ] All other clones deleted and re-cloned from the rewritten history — every clone that existed before 2026-07-20's rewrite (any other machine, WSL instance, or CI checkout) needs this
+- [ ] Every credential in the original inventory rotated (not just re-encrypted) — **the GitHub PAT found in history specifically still needs this**
+- [x] Pre-commit secret scanning hook added (`.githooks/pre-commit`, `gitleaks protect --staged`)
+- [x] `secrets-inventory.md` deleted from the working directory (never committed)