Ignore copilot agent branches in the CI

[llucax]

What's needed?

When using Copilot Coding Agent to create a PR, copilot pushes into a blessed repo branch, which triggers the CI workflow, but it will also be tested by the PR workflow, which is wasteful. We need to avoid this.

Proposed solution

1. Add copilot/** to the ignored branches in cookiecutter/{{cookiecutter.github_repo_name}}/.github/workflows/ci.yaml.

   name: Dispatch-CI
   
   on:
       # ...
       branches-ignore:
         - ...
         - 'copilot/**'

2. Add a migration step to cookiecutter/migrate.py to add the new ignore rule 'copilot/**' to existing repositories, but only if the rule doesn't exist already.

[llucax]

Actually, I think we should probably not allow creating copilot branches in blessed repos, for the same reason we don't allow creating any other WIP branches in blessed repos and require users to create their own branches in their own forks. At the end there is no real difference between a copilot branch and a user branch, we all don't need to see users chatting with an AI agent until a PR is ready for review.

Specially now that one can choose a different repo when assigning an issue to the copilot coding agent, there is pretty much no reason to create branches in blessed repos.