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/mcp_server.py

@@ -881,7 +881,7 @@ def handle_request(request):
         }
     elif method == "tools/call":
         tool_name = params.get("name")
-        tool_args = params.get("arguments", {})
+        tool_args = params.get("arguments") or {}
         if tool_name not in TOOLS:
             return {
                 "jsonrpc": "2.0",