fix(shellenv): propagate failed exit codes on Linux; fix e2e exit-code leak

[timvw]

Two pre-existing bugs found while working on #117.

1. The bash/zsh wrapper swallows failures on Linux

The wrapper runs commands under script(1) to give promptui a PTY. util-linux's script returns its own exit status unless --return is passed, so on Linux a failed command reported success to the shell:

wt co nonexistent-branch && echo "ok"   # printed ok

BSD/macOS script propagates the child's status, which is why this never showed up locally or in macOS CI. The fish integration already passes --return and even carries a comment explaining exactly this — bash and zsh were simply missed. Fix is the same one-flag change.

2. e2e/run.go leaks __exit_code between steps

${__exit_code:-0} only defaults when the variable is unset or empty, so a step expecting exit code 0 after a step expecting non-zero inherited the stale 1 and failed spuriously. This is why every non-zero assertion in the suite had to use skip_shellenv + $WT_BIN. Only the POSIX generator was affected — the fish and PowerShell generators assign the status unconditionally.

Regression coverage

Two new scenarios in shellenv.yaml, both exercised through the wrapper:

• shellenv_propagates_failure_exit_code — a failing wt remove exits 1.
• shellenv_success_after_failure_exit_code — a successful wt list after a failing command still reports 0.

Each fix is independently covered:

• Reverting only the --return change in a Linux container (golang:1.25, util-linux 2.41): 4 failures (both scenarios × bash and zsh). With the fix: 213 passed, 0 failed.
• Reverting only the run.go change on macOS: shellenv_success_after_failure_exit_code fails.

Full local suite on macOS: 316 passed, 0 failed. go vet and gofmt clean.

Note

--return is a util-linux long option. Busybox script does not support it, so a busybox-only environment would now error rather than silently return the wrong code — but the fish integration has already shipped this assumption, and environments without script at all (Git Bash, minimal containers) take the existing fallback branch, which captures $? correctly.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 36.98%. Comparing base (a81a147) to head (84045a0).

Files with missing lines	Patch %	Lines
e2e/run.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   36.82%   36.98%   +0.16%     
==========================================
  Files          28       28              
  Lines        3199     3201       +2     
==========================================
+ Hits         1178     1184       +6     
+ Misses       1935     1930       -5     
- Partials       86       87       +1     

Files with missing lines	Coverage Δ
cmd/shellenv.go	78.69% <100.00%> (+0.18%)	⬆️
e2e/run.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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