Epic: Implement 'project plan' command for backlog and sprint management

[duizendstra]

User Story

As a Project Maintainer / Scrum Master,
I want a CLI command to organize my GitHub issues into epics, stories, and tasks, and manage them within a GitHub Project board,
So that I can perform backlog grooming and sprint planning directly from my terminal.

Acceptance Criteria

• A new project plan command is created.
• The command can list all issues that are not currently assigned to an epic.
• The command provides an interactive way to assign one or more issues (stories/tasks) to a parent issue (an epic). This could be done by adding - [ ] #issue_number to the epics body.
• The command can list all available GitHub Project boards for the repository.
• The command provides an interactive way to add one or more issues to a specific project board.
• (Future) The command could potentially allow for moving issues between columns on a project board (e.g., "Todo", "In Progress").