create directory on startup (#2089)

## Motivation

<!-- Why is this change needed? What problem does it solve? -->
<!-- If it fixes an open issue, please link to the issue here -->

When you first run `uv run exo` you get an error like :

`FileNotFoundError: [Errno 2] No such file or directory:
'/Users/heidar/.exo/models'`

Manually tested on Macbook Pro M1 32GB

Fixes issue - https://github.com/exo-explore/exo/issues/2090

src/exo/main.py

@@ -18,7 +18,7 @@ from exo.download.impl_shard_downloader import exo_shard_downloader
 from exo.master.main import Master
 from exo.routing.event_router import EventRouter
 from exo.routing.router import Router, get_node_id_keypair
-from exo.shared.constants import EXO_LOG
+from exo.shared.constants import EXO_DEFAULT_MODELS_DIR, EXO_LOG
 from exo.shared.election import Election, ElectionResult
 from exo.shared.logging import logger_cleanup, logger_setup
 from exo.shared.types.common import NodeId, SessionId
@@ -71,6 +71,9 @@ class Node:
 
         logger.info(f"Starting node {node_id}")
 
+        # Errors the very first time exo is run as dir doesn't exist
+        EXO_DEFAULT_MODELS_DIR.mkdir(parents=True, exist_ok=True)
+
         # Create DownloadCoordinator (unless --no-downloads)
         if not args.no_downloads:
             download_coordinator = DownloadCoordinator(