fix: setup command branch naming and error handling

[fnebenfuehr]

Summary

• Fix branch-to-directory naming in setup: use branchToDirName() to convert slashes to dashes (e.g. chore/upgrade-next-16 → chore-upgrade-next-16)
• Fix spinner not stopping on error: wrap worktree.setup() with tryCatch and properly stop spinner with error message
• Update prompt to show correct target directory name

Test plan

• Run worktree setup on a branch with slashes (e.g. chore/foo) - should create chore-foo directory
• Trigger a setup error (e.g. directory exists) - spinner should stop and show error message

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