Expose `config_dir` in `add_marketplace` MCP tool

https://github.com/yusufkaraaslan/Skill_Seekers/blob/f3972efa33fa79634b96936acf1fac321cdcf7c1/src/skill_seekers/services/marketplace_manager.py#L15

• Expose this parameter in the MCP tool framework so that "local", "project", and/or "global" marketplaces can be setup.
• Give the above an order of precedence.