Add worktree awareness to PR checkout

[nchudleigh]

Summary

• Before running gh pr checkout, check if the PR's branch is already checked out in a git worktree
• If a matching worktree is found, skip the checkout and cd directly to that path instead
• Uses git worktree list --porcelain to find worktrees by branch name

Test plan

• Select a PR whose branch is checked out in a worktree — should print "Branch '...' already checked out at ..." and cd there
• Select a PR with no existing worktree — should behave as before