fix marketplace schema validation error in dev-workflow plugin

Remove invalid skills path that started with "../" instead of required "./" prefix.
The codex skill is already available as a standalone plugin, so dev-workflow can call it directly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -255,9 +255,6 @@
       ],
       "agents": [
         "./agents/dev-plan-generator.md"
-      ],
-      "skills": [
-        "../skills/codex/SKILL.md"
       ]
     }
   ]