feat(doctor): nomarchy-doctor — read-only health sheet + VM check (item 10)

The things that actually break user machines, one command (and menu ›
System › Doctor): failed system/user units, disk space (real
filesystems only — fstype allowlist makes it VM/live safe, device
dedupe collapses / and /nix), theme-state.json parses + is
git-tracked, flake dirty/behind as warnings, generation age (profile
symlink mtime — store paths are epoch-1), snapper timeline when
enabled. Read-only by contract; every ✖ prints the one fixing command;
exit 1 on any problem. Ships as pkgs/nomarchy-doctor so the VM check
runs on a minimal node.

Verified: V1 package build (shellcheck-gated) + a real-hardware smoke
that correctly flagged a genuinely failed user unit on the dev box;
V2 green — checks.doctor VM test: induced failed unit flips the sheet
to exit-1 naming the unit and its fix, reset-failed returns healthy.

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

modules/nixos/default.nix

@@ -293,6 +293,7 @@ in
     # ── Essential packages ───────────────────────────────────────────
     environment.systemPackages = with pkgs; [
       nomarchy-theme-sync # provided by overlays.default
+      nomarchy-doctor     # read-only health check (System › Doctor)
 
       # Friendly wrappers for the two rebuild paths (README §3). Run as
       # your user: `nix flake update` must NOT run as root (libgit2