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.

agent/CONVENTIONS.md

@@ -24,7 +24,7 @@ those, it probably shouldn't exist.
 
 ## Feature design
 - **In-flake state:** any user-settable config gets a menu writer that
-  lands it in `theme-state.json` (`settings.*`), git-tracked. No
+  lands it in `state.json` (`settings.*`), git-tracked. No
   `~/.local/state`, no side files. Instant-effect where possible
   (`--no-switch` + flip the service; the service reads the *live* working
   tree at start — the night-light `ExecCondition` pattern). Rebuild-baked