Separate ChromaDB model cache into its own named volume

The ONNX embedding model (~79 MB) downloads to ~/.cache/chroma/ on
first mempalace search. Without persistence it re-downloads on every
container recreation. Add a separate devbox-chroma-cache volume
rather than mixing it into the palace data volume — model cache is
disposable (delete and re-download), palace data is precious (back
up and migrate). Both volumes are commented out by default (opt-in).

Updated README.md storage section to explain the two-volume split
and the air-gapped pre-population path. Added chroma cache row to
DOCKER_HUB.md data storage table.

docker-compose.shared.yml

@@ -64,6 +64,9 @@ services:
       # Optional: persist MemPalace data (conversation memory, knowledge graph)
       # - devbox-palace:/home/developer/.mempalace
 
+      # Optional: persist ChromaDB embedding model cache (~79 MB)
+      # - devbox-chroma-cache:/home/developer/.cache/chroma
+
       # Optional: AWS credentials (per-user if available)
       # - ${HOME}/${SIGNUM}/.aws:/home/developer/.aws
 
@@ -74,3 +77,4 @@ volumes:
   devbox-nvim-data:
   devbox-uv:
   # devbox-palace:
+  # devbox-chroma-cache: