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/normalize.py

@@ -26,6 +26,9 @@ def normalize(filepath: str) -> str:
     Plain text files pass through unchanged.
     """
     try:
+        file_size = os.path.getsize(filepath)
+        if file_size > 500 * 1024 * 1024:  # 500 MB safety limit
+            raise IOError(f"File too large ({file_size // (1024*1024)} MB): {filepath}")
         with open(filepath, "r", encoding="utf-8", errors="replace") as f:
             content = f.read()
     except OSError as e: