feat(tests): add more tests, run env-changing tests serially

[sudokoi]

Add serial_test dev-dependency and annotate environment/current-dir mutating tests in ests/paths_test.rs with #[serial] to prevent parallel test interference.

[codecov]

Codecov Report

❌ Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.69%. Comparing base (6843b86) to head (445a6f0).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/prompt.rs	50.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #3       +/-   ##
===========================================
+ Coverage   35.28%   57.69%   +22.40%     
===========================================
  Files          10       10               
  Lines         700      728       +28     
===========================================
+ Hits          247      420      +173     
+ Misses        453      308      -145     

Flag	Coverage Δ
unittests	57.69% <53.84%> (+22.40%)	⬆️

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[sudokoi]

@codecov-ai-reviewer test