Make beads claude code skill installable via marketplace (#468)

Reorganize the claude-code-skill into a publishable plugin that can be
installed through the beads-marketplace. Users can now reference the
'beads' skill right after installing the marketplace.

Co-authored-by: Steve Yegge <steve.yegge@gmail.com>

.claude-plugin/marketplace.json

@@ -9,7 +9,7 @@
       "name": "beads",
       "source": "./",
       "description": "AI-supervised issue tracker for coding workflows",
-      "version": "0.29.0"
+      "version": "0.30.0"
     }
   ]
 }