Shell selection ignores user's default shell

[knowsuchagency]

Description

Currently, Vibe Kanban hardcodes a preference for zsh over bash when selecting which shell to use for command execution. This behavior ignores the user's configured default shell set in the SHELL environment variable.

Current Behavior

The get_shell_command() function in crates/utils/src/shell.rs checks for shells in this order:

1. zsh (if /bin/zsh exists)
2. bash (if /bin/bash exists)
3. sh (fallback)

This means users with bash as their default shell will still get zsh if it's installed, leading to inconsistent behavior.

Expected Behavior

Vibe Kanban should respect the user's SHELL environment variable, which reflects their configured default shell, and only fall back to hardcoded preferences if that variable is not set or invalid.

Impact

Users may experience:

• Unexpected shell behavior
• Inconsistent environment across system commands
• Issues with shell-specific configurations not being honored