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/GOALS.md

@@ -14,7 +14,7 @@ confidence, default identity, release bar) live in
    on-hardware QA.
 2. **Reproducible, with zero hidden state.** The downstream flake checkout
    *is* the machine. All user-settable config is menu-writable into the
-   git-tracked state file (`theme-state.json` `settings.*`) — never
+   git-tracked state file (`state.json` `settings.*`) — never
    `~/.local/state`, never `~/.config` side files. Re-cloning your flake
    reproduces the machine, settings and all.
 3. **Effortless to configure.** The user never has to learn Nix. Every