feat(halo): serve bge-m3 embeddings alongside coder

Add a multilingual bge-m3 embedding model to the llama-server preset and
raise --models-max to 2 so it stays co-resident with the coder model.
This gives the RAG stack a local embeddings endpoint without a second
service, keeping all inference on halo. Embedding-specific overrides
(ubatch-size, context, pooling) are pinned since the global defaults
would truncate or misconfigure embedding requests.

systems/x86_64-linux/halo/llama-server.nix

@@ -29,7 +29,7 @@
         "--host 0.0.0.0"
         "--port 8000"
         "--models-preset ${./models.ini}"
-        "--models-max 1"
+        "--models-max 2"
       ];
       Restart = "on-failure";
       RestartSec = 10;