This commit introduces a new plugin for integrating with GitHub Issues.
The `issue` plugin provides the following commands:
- `/core:issue list`: List open issues.
- `/core:issue view <number>`: View the details of a specific issue.
- `/core:issue start <number>`: Start working on an issue by creating a feature branch.
- `/core:issue close <number>`: Close an issue by creating a pull request.
The plugin uses the GitHub CLI (`gh`) to interact with the GitHub API.
