[FEATURE] Allow nushell for default shell

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

I'm a nushell user for all platforms I use (Windows/Linux/macOS) and would like to stick to it. Bash commands that Claude Code emits are hardly readable to me.

Proposed Solution

Allow nushell for defaultShell

Alternative Solutions

(Beg claude to use nushell every time, make some relevant skill and see how it's just ignored)

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

More readable commands so that I can be more confident to allow.

Additional Context

Previous: #58312