fix: MCP null args hang, repair infinite recursion, OOM on large files

Three critical bugfixes:

1. MCP server hangs on null arguments (#394) — `params.get("arguments", {})`
   returns None when JSON has `"arguments": null`. Changed to `or {}`.

2. cmd_repair infinite recursion (#395) — trailing slash on palace_path
   caused backup_path to be inside the source dir. Strip trailing sep.

3. OOM on large transcript files (#396) — split_mega_files.py and
   normalize.py load entire files into memory. Added 500MB safety limit
   with clear skip/error messages.

Closes #394, #395, #396.

mempalace/cli.py

@@ -202,6 +202,7 @@ def cmd_repair(args):
     print(f"  Extracted {len(all_ids)} drawers")
 
     # Backup and rebuild
+    palace_path = palace_path.rstrip(os.sep)
     backup_path = palace_path + ".backup"
     if os.path.exists(backup_path):
         shutil.rmtree(backup_path)