feat: add pr checkout command

[zoldyzdk]

Summary

• Adds bb pr checkout <pr-id> command that fetches a PR's source branch and checks it out locally
• Creates a git operations module (cmd/git_ops.go) with gitFetch and gitCheckout helpers using os/exec
• Supports --branch / -b to override the local branch name (defaults to PR source branch)
• Supports --force / -f to reset an existing local branch to the latest PR state

Closes #3

Test plan

• bb pr checkout <id> fetches and checks out the PR's source branch
• Local branch defaults to the PR's source branch name
• --branch my-name uses custom local branch name
• --force resets an existing branch
• Invalid PR ID shows clear error
• Missing auth shows clear error

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