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.

agent/BACKLOG.md

@@ -121,6 +121,18 @@ size, or truncation. Either fix the guest (fontconfig + icon cache in the
 test node) or document the gap in docs/TESTING.md §5 so the next agent does
 not chase it — right now nothing warns them.
 
+### 133. No check pins the #107 legacy-name compat shim
+
+`lib.mkFlake`, doctor and lifecycle all still accept `theme-state.json`, and
+`nomarchy-state-sync` migrates it on write — but nothing in `checks.*`
+evaluates a checkout that has *only* the legacy name, so the shim can rot
+silently while every check stays green. It is load-bearing until every
+existing machine has taken one menu write. Add a cheap eval check: a fixture
+dir with `theme-state.json` only → `mkFlake` resolves it; plus the write-side
+migration (state.json created + git-staged, legacy `git rm`'d). Proven by
+hand 2026-07-15 (drvPaths identical across the rename) — this just keeps it
+proven. Delete together with the shim + the `nomarchy-theme-sync` alias.
+
 ### 120. A netinstall ISO, next to the fat offline one
 
 Bernardo 2026-07-14, after seeing the measured size: **keep the current ISO