## Motivation
Replace confusing EXO_MODELS_DIR/EXO_MODELS_PATH with clearer
multi-directory support, enabling automatic download spillover across
volumes.
## Changes
- EXO_MODELS_DIRS: colon-separated writable dirs (default always
prepended, first with enough space wins)
- EXO_MODELS_READ_ONLY_DIRS: colon-separated read-only dirs (protected
from deletion)
- select_download_dir(): picks writable dir by free space
- resolve_existing_model(): unified lookup across all dirs
- is_read_only_model_dir(): path-based read-only detection instead of
hardcoded flag
- Updated coordinator, worker, model cards, tests
## Why It Works
Default dir always included so zero-config behavior is unchanged. Disk
space checked at download time for automatic spillover. Read-only status
derived from path, not hardcoded.
## Test Plan
### Manual Testing
- No env vars set → identical behavior
- EXO_MODELS_DIRS=/Volumes/SSD/models → downloads to external storage
- EXO_MODELS_READ_ONLY_DIRS=/mnt/nfs → models found, deletion blocked
### Automated Testing
- 4 new tests in test_xdg_paths.py (prepend, default-only, overlap,
empty read-only)
- Existing tests updated to patch new constants
