Guard against infinite loop when no training shards exist, fix README typo

Add assertion after filtering val_path from parquet_paths for the "train"
split so an empty list fails fast instead of spinning in a silent infinite
loop. Also remove stray article "a" in README ("a three files" → "three
files").

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -8,7 +8,7 @@ The idea: give an AI agent a small but real LLM training setup and let it experi
 
 ## How it works
 
-The repo is deliberately kept small and only really has a three files that matter:
+The repo is deliberately kept small and only really has three files that matter:
 
 - **`prepare.py`** — fixed constants, one-time data prep (downloads training data, trains a BPE tokenizer), and runtime utilities (dataloader, evaluation). Not modified.
 - **`train.py`** — the single file the agent edits. Contains the full GPT model, optimizer (Muon + AdamW), and training loop. Everything is fair game: architecture, hyperparameters, optimizer, batch size, etc. **This file is edited and iterated on by the agent**.

prepare.py

@@ -258,6 +258,7 @@ def _document_batches(split, tokenizer_batch_size=128):
     val_path = os.path.join(DATA_DIR, VAL_FILENAME)
     if split == "train":
         parquet_paths = [p for p in parquet_paths if p != val_path]
+        assert len(parquet_paths) > 0, "No training shards found."
     else:
         parquet_paths = [val_path]
     epoch = 1