fix: respect ZDOTDIR for zsh init

[brosu]

wt init zsh now updates ${ZDOTDIR:-$HOME}/.zshrc (and activation instructions match), instead of always writing ~/.zshrc.

Tests:

• go test ./... (includes TestGetShellConfigPathZshRespectsZdotdir and updated TestGetShellConfigPath to be stable when ZDOTDIR is set)

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 27.38%. Comparing base (0c58a2c) to head (1ffd3d9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
init.go	33.33%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   27.19%   27.38%   +0.18%     
==========================================
  Files           4        4              
  Lines        1570     1574       +4     
==========================================
+ Hits          427      431       +4     
  Misses       1094     1094              
  Partials       49       49              

Files with missing lines	Coverage Δ
init.go	45.06% <33.33%> (+0.12%)	⬆️

... and 1 file with indirect coverage changes

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[timvw]

Looks good to me. Approving and merging.

[timvw]

Thanks a lot for the fix and tests, really appreciate it 🙌