meta_005568.json: add user_config so chat_sft inherits the run's LRs

meta_001950.json: add user_config so chat_sft inherits the run's LRs

Add nanochat-format d24 checkpoint (step 5568) + world_size=8 optimizer shards

Correct optimizer-state notes: SFT needs only the weights; W_O axis is a no-op at d24

Add model card, meta.json, logs, tokenizer, training source, and nanochat converter