sketchdev mcp: fix missing env variable for MCP substitution
diff --git a/cmd/sketch/main.go b/cmd/sketch/main.go
index 103ebc4..74571f7 100644
--- a/cmd/sketch/main.go
+++ b/cmd/sketch/main.go
@@ -639,6 +639,7 @@
 	// This is needed for MCP server authentication placeholder replacement
 	if pubKey != "" {
 		os.Setenv("SKETCH_PUB_KEY", pubKey)
+		os.Setenv("SKETCH_MODEL_API_KEY", apiKey)
 	}
 
 	wd, err := os.Getwd()