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.

templates/downstream/README.md

@@ -15,19 +15,19 @@ keyboard, detected hardware, password hash, …). Keep commented opt-ins and
    services — the login user is created from `username` automatically) and
    `home.nix` (your packages).
 3. `git init && git add -A` — flakes only see tracked files, including
-   `theme-state.json`.
+   `state.json`.
 4. System: `sudo nixos-rebuild switch --flake .#default`
 5. Desktop: `nix run home-manager -- switch --flake .#me`
    (afterwards just `home-manager switch --flake .#me` — it's installed)
 6. Clone/symlink this directory to `~/.nomarchy` (or export
-   `NOMARCHY_PATH`) so `nomarchy-theme-sync` knows where the state lives.
+   `NOMARCHY_PATH`) so `nomarchy-state-sync` knows where the state lives.
 
 Day-to-day:
 
 ```sh
-nomarchy-theme-sync list             # 24 shipped presets
-nomarchy-theme-sync apply gruvbox    # writes state + home-manager switch
-nomarchy-theme-sync bg next          # cycle wallpapers (instant, no rebuild)
+nomarchy-state-sync list             # 24 shipped presets
+nomarchy-state-sync apply gruvbox    # writes state + home-manager switch
+nomarchy-state-sync bg next          # cycle wallpapers (instant, no rebuild)
 ```
 
 The system layer only needs `nixos-rebuild` when you change `system.nix`.