feat: atuin hex support $SHELL

[xav-ie]

Greetings, I am open to an alternative solution like subcommand support or something else which I am happy to accomodate.

Fixes #3326.

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[greptile-apps]

Greptile Summary

Small fix that prefers $SHELL over CommandBuilder::new_default_prog() when spawning the user's shell in atuin hex, with a fallback to the original behavior when $SHELL is unset.

• The approach is correct and idiomatic for Unix systems; Windows users unaffected since $SHELL is typically unset there.
• One minor edge case: $SHELL="" (set but empty) reaches CommandBuilder::new("") and fails at spawn with an opaque error rather than falling back gracefully.

Important Files Changed

Filename	Overview
crates/atuin-hex/src/lib.rs	Uses $SHELL env var to spawn the user's preferred shell instead of always relying on new_default_prog(). One minor edge case: if $SHELL is set to an empty string, CommandBuilder::new("") will be called and fail at spawn time.

_{Reviews (1): Last reviewed commit: "feat(atuin hex): allow subshell spawn to..." | Re-trigger Greptile}

[ellie]

Thank you for the work here! I'm not sure this makes sense to override it - typically, $SHELL is set to the login shell, and while it could make sense to allow the user to override it to change what hex launches, it may then affect anything running within hex that might expect that to remain true

In this case, I think we'd be better off allowing the user to provide --shell to hex, and pass in a path to launch

[xav-ie]

That sounds good to me. Let me try to update this PR

[limonkufu]

Hey any progress on this or a workaround for nushell?