Function write_activate_snippets 

pub fn write_activate_snippets() -> Result<(PathBuf, PathBuf)>

Write activation snippets to ~/.mlstack/global/.

Creates:

• activate-global.sh (bash/zsh): export PATH="$HOME/.mlstack/global/bin:$PATH"
• activate-global.fish (fish): fish_add_path $HOME/.mlstack/global/bin

Returns (sh_path, fish_path).