Make use of nushell's autoload directory

[surma]

Summary

Changes shadowenv init nushell to automatically install the shell hook into nushell's autoload directory instead of printing a script for users to manually paste into their config.

Changes

• src/init.rs: Added install_nushell_hook() function that:

  • Queries nushell for its autoload directory via nu -c '$nu.user-autoload-dirs | first'
  • Creates the autoload directory if it doesn't exist
  • Writes shadowenv.nu with the hook script
  • Returns Result<()> for proper error handling

• src/main.rs: Updated to handle the new return type from init::run()

• README.md: Updated nushell instructions to reflect the new behavior

Behavior

Before:

$ shadowenv init nushell
# prints script to stdout, user must paste into config.nu

After:

$ shadowenv init nushell
Wrote shadowenv hook to /Users/you/Library/Application Support/nushell/autoload/shadowenv.nu

The autoload directory is queried from nushell itself, so it works correctly across platforms (macOS, Linux) and respects any custom configuration.