Support gh secret set --app agents #13419

Describe the feature or problem you’d like to solve

Today I can set org secrets for actions, codespaces and dependabot.
For a few days it is now also possible to set secrets for agents.

Proposed solution

Allow agents as application for a secret.

Additional context

https://github.blog/changelog/2026-05-08-more-flexible-secrets-and-variables-for-copilot-cloud-agent/
https://docs.github.com/en/rest/agents/secrets?apiVersion=2026-03-10#create-or-update-an-organization-secret