Clarify README editing guidance

Co-authored-by: Ishaan Jaff <ishaan-jaff@users.noreply.github.com>

Author: cursoragent

.evidence/after.png

@@ -9,5 +9,5 @@ npm run dev
 
 ## Notes
 
-- [App.svelte](https://github.com/fastrepl/contextlengthof/blob/main/src/App.svelte) is all you need to edit.
+- Start with [App.svelte](https://github.com/fastrepl/contextlengthof/blob/main/src/App.svelte) for most UI changes.
 - [PicoCSS](https://picocss.com/) is used for styling.

.evidence/before.png

@@ -8,6 +8,7 @@
     "build": "vite build",
     "preview": "vite preview",
     "check": "svelte-check --tsconfig ./tsconfig.json",
+    "check:readme": "node scripts/check-readme-phrasing.mjs",
     "format": "prettier --write ."
   },
   "devDependencies": {

README.md

@@ -0,0 +1,11 @@
+import { readFileSync } from 'node:fs';
+
+const readme = readFileSync(new URL('../README.md', import.meta.url), 'utf8');
+const unclearPhrase = 'is all you need to edit';
+
+if (readme.includes(unclearPhrase)) {
+  console.error(`README uses overly broad guidance: "${unclearPhrase}"`);
+  process.exit(1);
+}
+
+console.log('README phrasing check passed.');