Add 'handoff' skill to improve workflow documentation and agent transitions

.claude-plugin/plugin.json

@@ -13,6 +13,7 @@
     "./skills/engineering/prototype",
     "./skills/productivity/caveman",
     "./skills/productivity/grill-me",
+    "./skills/productivity/handoff",
     "./skills/productivity/write-a-skill"
   ]
 }