fix: plugin provides local core-agent MCP (not remote mcp.lthn.sh)

Plugin's mcp.json now uses local "core-agent mcp" via stdio instead of
remote HTTPS. This means installing the plugin auto-registers the MCP
server — no manual .mcp.json needed.

Co-Authored-By: Virgil <virgil@lethean.io>

claude/core/mcp.json

@@ -1,12 +1,9 @@
 {
   "mcpServers": {
     "core": {
-      "type": "http",
-      "url": "https://mcp.lthn.sh",
-      "headers": {
-        "Authorization": "Bearer $CORE_BRAIN_KEY"
-      },
-      "allowedEnvVars": ["CORE_BRAIN_KEY"]
+      "type": "stdio",
+      "command": "core-agent",
+      "args": ["mcp"]
     }
   }
 }