refactor(#107): theme-state.json → state.json, theme-sync → state-sync

The machine flake's git-tracked settings file is system state, not
"theme" only — rename it to state.json. CLI becomes nomarchy-state-sync
with a nomarchy-theme-sync symlink for scripts and muscle memory.

Eval (mkFlake, doctor, lifecycle) still accepts theme-state.json; the
next write migrates to state.json and removes the legacy file.
Documented in MIGRATION.md; drop the CLI alias after release notes.

modules/nixos/timezone.nix

@@ -4,7 +4,7 @@
 # /etc/localtime at runtime.
 #
 # In-flake state, menu-driven (the keyboard/night-light philosophy): the on/off
-# flag lives in the same theme-state.json under `settings.autoTimezone`
+# flag lives in the same state.json under `settings.autoTimezone`
 # (git-tracked, reproducible), written by the System-menu toggle
 # (nomarchy-autotimezone). Because this is a SYSTEM service — not a user unit it
 # can start/stop instantly like night-light — the toggle drives a system rebuild
@@ -16,14 +16,14 @@ let
 
   # Read the same state file the rest of the system side uses (Plymouth too),
   # wired by lib.mkFlake. The flag defaults off when stateFile is null;
-  # a set-but-missing/invalid path fails closed via theme-state-read.nix.
+  # a set-but-missing/invalid path fails closed via state-read.nix.
   state =
     if cfg.stateFile != null
-    then import ../theme-state-read.nix { inherit lib; } cfg.stateFile
+    then import ../state-read.nix { inherit lib; } cfg.stateFile
     else { };
   stateEnabled = (state.settings or { }).autoTimezone or false;
 
-  sync = lib.getExe pkgs.nomarchy-theme-sync;
+  sync = lib.getExe pkgs.nomarchy-state-sync;
 
   # Menu/CLI toggle. Runs as the normal user (it owns the flake checkout +
   # writes the state); sudos only the system switch, like sys-update. Writes