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/home/osd.nix

@@ -2,7 +2,7 @@
 # lock). The media keys drive it via swayosd-client (it performs the
 # action AND shows the OSD), so changing volume or brightness gives the
 # visual feedback that bare wpctl/brightnessctl didn't. Themed from
-# theme-state.json. Brightness needs the backlight udev rule shipped
+# state.json. Brightness needs the backlight udev rule shipped
 # system-side (modules/nixos/default.nix → services.udev.packages).
 { config, lib, pkgs, ... }: