[codex] warn on case-insensitive worktree collisions

[timvw]

Summary

• Make worktree lookup fall back to case-insensitive branch matching on case-insensitive filesystems.
• Warn before creating/checking out a worktree when the computed path collides with an existing path component by case.
• Document the macOS case-insensitive filesystem gotcha and the separator = "-" mitigation with neutral examples.

Validation

• go test ./...
• go build -o /tmp/wt-build-check .
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 45.12195% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.21%. Comparing base (39b8446) to head (8e44ec2).

Files with missing lines	Patch %	Lines
cmd/worktree_path.go	36.76%	40 Missing and 3 partials ⚠️
cmd/checkout.go	0.00%	1 Missing ⚠️
cmd/create.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   28.01%   30.21%   +2.19%     
==========================================
  Files          27       27              
  Lines        3352     2787     -565     
==========================================
- Hits          939      842      -97     
+ Misses       2334     1859     -475     
- Partials       79       86       +7     

Files with missing lines	Coverage Δ
cmd/repo.go	65.06% <100.00%> (+7.91%)	⬆️
cmd/checkout.go	0.00% <0.00%> (ø)
cmd/create.go	0.00% <0.00%> (ø)
cmd/worktree_path.go	58.28% <36.76%> (-10.69%)	⬇️

... and 22 files with indirect coverage changes

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