fix: replace Unicode separator in convo_miner.py for Windows compatibility

Replace the ─ (U+2500) separator character with - in convo_miner.py.
Windows terminals using cp1252 encoding raise UnicodeEncodeError when
printing this character unless PYTHONUTF8=1 is set explicitly.

Fixes crash on Windows: UnicodeEncodeError: 'charmap' codec can't encode
character '\u2500'

mempalace/convo_miner.py

@@ -286,7 +286,7 @@ def mine_convos(
     print(f"  Palace:  {palace_path}")
     if dry_run:
         print("  DRY RUN — nothing will be filed")
-    print(f"{'─' * 55}\n")
+    print(f"{'-' * 55}\n")
 
     collection = get_collection(palace_path) if not dry_run else None