fix: remove worktree directories after removal

[timvw]

Summary

• delete the worktree directory after git worktree remove completes
• prune the repo folder under WORKTREE_ROOT if it is left empty
• add regression test covering cleanup when git leaves the path behind

[codecov]

Codecov Report

❌ Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 18.95%. Comparing base (099dbae) to head (0a64792).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
main.go	0.00%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   20.17%   18.95%   -1.23%     
==========================================
  Files           1        1              
  Lines         466      496      +30     
==========================================
  Hits           94       94              
- Misses        366      396      +30     
  Partials        6        6              

Files with missing lines	Coverage Δ
main.go	18.95% <0.00%> (-1.23%)	⬇️

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