docs: clarify Nushell XDG startup requirements and recovery

[naokihaba]

resolves: #2491

Context

Nushell sets up its autoload directories before config.nu runs. If config.nu then changes XDG_DATA_HOME to a different location, the Vite+ installer picks up the new path and writes its snippet to a directory that fresh sessions launched from the original parent environment will not load. If XDG_DATA_HOME is never set, this is not an issue.

This setup does not meet Nushell's documented startup requirement, which states that directory-related XDG variables must be set before Nushell launches. While assigning them in config.nu is valid, it only affects child processes and will not update the autoload paths already resolved for the current session.

As described in the investigation results, we reproduced the mismatch on macOS with Nushell 0.115.1. Passing the original autoload path explicitly would require changing how the installer is invoked, switching to user autoload only addresses part of the problem, and warning whenever XDG_DATA_HOME is set would be noisy for correctly configured users.

Based on those findings, this PR updates the Getting Started guide by documenting the startup requirement and outlining a workaround for affected users, who can manually source Vite+'s env.nu in their config.nu. We have left the installer itself untouched.

Integration tests reproduce the mismatch, verify that everything works when XDG_DATA_HOME is set before startup, and confirm that sourcing the file allows a fresh session to load Vite+ and run vp help.