fix(state): the "rename it" tip led into the error it was fixing

Verifying Bernardo's #107 upgrade path (legacy theme-state.json checkout →
new code) turned up a footgun in state-read.nix's own help text. It said:

  • Still on theme-state.json? Rename it to state.json (…)

On a git flake only tracked files exist, so a hand-rename leaves state.json
untracked and therefore invisible — producing exactly the "state file is
missing" error the tip hangs off. Point at the menu write (which stages the
rename via git add --intent-to-add + git rm), name `git add state.json` for
the manual path, and add a bullet for "file is on disk but eval says
missing", which is otherwise a genuinely confusing dead end.

Message verified by rendering the real failure, not by reading the source.

The rename itself is safe and needs no code change: with identical content,
theme-state.json and state.json produce byte-identical derivations (HM
2yqqy8h…, system 9qp9l3g…), so it cannot change what boots. Legacy eval,
the migration write, and re-eval on a dirty/intent-to-add tree all pass.
#133 filed: no check pins the compat shim, so it can rot before we drop it.

modules/state-read.nix

@@ -28,8 +28,12 @@ let
       • Field-level schema (colors, ui, border, …):
           nomarchy-state-sync validate
         Eval-time field checks live in modules/home/theme.nix.
-      • Still on theme-state.json? Rename it to state.json (or just run
-        any menu write / `nomarchy-state-sync set` — it migrates).'';
+      • Still on theme-state.json? Easiest is any menu write /
+        `nomarchy-state-sync set` — it migrates and stages the rename
+        for you. Renaming by hand needs `git add state.json` too.
+      • File IS on disk but eval still says missing? On a git flake only
+        *tracked* files exist — an untracked state.json is invisible:
+          git add state.json'';
 
   missingMsg = ''
 
@@ -37,7 +41,7 @@ let
       ${pathStr}
 
     This file is required (appearance + menu settings). Add state.json to
-    your flake checkout so evaluation stays pure.
+    your flake checkout — git-tracked — so evaluation stays pure.
     ${tip}'';
 
   emptyMsg = ''