Clarify README app entry note

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

Author: cursoragent

.evidence/readme-before.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.
+- [App.svelte](https://github.com/fastrepl/contextlengthof/blob/main/src/App.svelte) contains the main app code.
 - [PicoCSS](https://picocss.com/) is used for styling.

README.md

@@ -0,0 +1,20 @@
+from pathlib import Path
+
+
+readme = Path(__file__).resolve().parents[1] / "README.md"
+content = readme.read_text()
+
+expected = (
+    "[App.svelte](https://github.com/fastrepl/contextlengthof/blob/main/src/App.svelte) "
+    "contains the main app code."
+)
+
+if expected not in content:
+    raise SystemExit(
+        "README should describe App.svelte as containing the main app code."
+    )
+
+if "is all you need to edit" in content:
+    raise SystemExit("README should not imply App.svelte is always the only file to edit.")
+
+print("PASS: README clearly describes App.svelte.")