Add copilot harness support for gcp-mcp-setup skill

[antoinemerc]

The gcp-mcp-setup skill is not compatible with the Copilot CLI harness (or any other harness).

• Add Copilot CLI support to the gcp-mcp-setup skill in a similar way to how other harnesses are handled, with a harness flag to target the correct config path and format.
• Update documentation and compatibility table to reflect the new support.

Support also needs to be added for the verify script

[github-actions]

🤖 Hi @antoinemerc, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.