test: add bats tests for create_worktree

[RodrigoEspinosa]

🎯 What: Added comprehensive test coverage for the create_worktree function in bin/wt, which was missing tests.
📊 Coverage: Covered 4 primary scenarios: early return if the target directory exists, creating a new branch and worktree, checking out an existing branch into a worktree, and checking out a new branch from a specified start point.
✨ Result: Improved test coverage and reliability for worktree creation operations. Tested via bats tests/.

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PR created automatically by Jules for task 8125130983199600728 started by @RodrigoEspinosa

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