fix: specify UTF-8 encoding when reading subgraph files

On Windows, Python defaults to cp1252 encoding when no encoding is
specified. JSON files containing UTF-8 characters (e.g., non-ASCII
characters) cause UnicodeDecodeError when read with cp1252.

This fixes the error that occurs when loading blueprint subgraphs
on Windows systems.

https://claude.ai/code/session_014WHi3SL9Gzsi3U6kbSjbSb

app/subgraph_manager.py

@@ -53,7 +53,7 @@ class SubgraphManager:
         return entry_id, entry
 
     async def load_entry_data(self, entry: SubgraphEntry):
-        with open(entry['path'], 'r') as f:
+        with open(entry['path'], 'r', encoding='utf-8') as f:
             entry['data'] = f.read()
         return entry