Add cloving generate pull-request command

[cardmagic]

Add a new cloving generate pull-request command that generates a pull request description based on the current branch's changes.

Proposed functionality

• Analyze commits and diff between current branch and base branch (main/master)
• Generate a structured PR description with:
  • Summary of changes
  • Test plan
  • Any breaking changes or migration notes
• Support for different PR templates
• Option to create the PR directly via gh pr create

Related

Similar to how cloving commit generates commit messages from staged changes.