Update README to reference base model branch

Replace Checkpoint-Two references with base model documentation. Update
repository link from tplr/Covenant72B Checkpoint-Two branch to
1Covenant/Covenant72B base branch.

- Add new base.webp image asset
- Update README model files link to base branch
- Change wording from checkpoint-specific to base model
- Update image reference from checkpoint-two.webp to base.webp

README.md

@@ -12,12 +12,11 @@ model trained entirely from scratch at the 72 billion parameter scale.
 It is being trained with 20+ globally distributed participants coordinated via
 decentralized infrastructure on the Bittensor blockchain.
 
-**Checkpoint-Two** marks the second release, corresponding to **420 billion
-tokens processed**. Model files are available in the [Checkpoint-Two
-branch](https://huggingface.co/tplr/Covenant72B/tree/Checkpoint-Two). Future
+The latest base model files are available on the
+[base branch](https://huggingface.co/1Covenant/Covenant72B/tree/base). Future
 checkpoints will be updated here.
 
-![Checkpoint Two](assets/checkpoint-two.webp)
+![Covenant72B](assets/base.webp)
 
 ---