Compatibility issues when using Nushell as default shell

[frieser]

When using Nushell (nu) as the default shell, workmux encounters several issues:

1. Syntax Incompatibility: Nushell does not support POSIX redirections like 2>/dev/null in the same way, causing the PaneHandshake wrapper to fail or show errors.
2. Command Rewriting: The current mechanism for injecting prompt files uses "$(cat ...)", which is invalid syntax in Nushell.
3. Race Conditions: In some cases, characters sent via send-keys (specifically the Enter key) are lost because they arrive too quickly during the shell's interactive startup phase.