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.

tools/import-palettes.py

@@ -131,7 +131,7 @@ def convert(theme_dir: Path) -> dict:
         "name": slug.replace("-", " ").title(),
         "slug": slug,
         "mode": mode,
-        # Empty = auto: nomarchy-theme-sync falls back to the first file in
+        # Empty = auto: nomarchy-state-sync falls back to the first file in
         # the theme's backgrounds/ directory.
         "wallpaper": "",
         "colors": {