build: add numpy-stl + Open3DModel config block for find_model

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -8,3 +8,8 @@ OPENROUTER_API_KEY=sk-or-v1-XXXX
 
 # Porta HTTP del servizio (default 8890).
 PRESA_MANO_PORT=8890
+
+# Chiave API per Open3DModel (pose-fit 6DOF su STL).
+# Generala con la procedura del README di Open3DModel; deve combaciare
+# con OPEN3D_API_KEY in ./Open3DModel/.env.
+OPEN3D_API_KEY=