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/MEMORY.md

@@ -100,10 +100,10 @@ iteration would otherwise rediscover.
 - **NixOS release bump is a trap:** the discarded attempt
   (branch deleted 2026-06-22) hit a Hyprland OOM blocker; a redo is a
   deliberate `v2`, never part of routine lock bumps.
-- `theme-state.json` is git-tracked inside an 86 MB flake tree, so every
+- `state.json` is git-tracked inside an 86 MB flake tree, so every
   state write re-copies the source before eval — the wallpapers-artifact
   split (BACKLOG LATER) is the decided fix (§ Faster switches).
-- **Friendly theme-state load** (`modules/theme-state-read.nix`, #66):
+- **Friendly theme-state load** (`modules/state-read.nix`, #66):
   `builtins.tryEval` does **not** catch `readFile`/`fromJSON` failures —
   gate with `pathExists` + empty/non-object checks before `fromJSON`.
   Subtle JSON syntax errors still surface from nlohmann (line/col);