Add shell completions generation (bash, zsh, fish)

[thomasnemer]

Problem

No shell completions are provided. Users must type full command and argument names without tab-completion assistance.

Suggested Fix

clap supports built-in shell completion generation via the clap_complete crate. Add a hidden syld completions <shell> subcommand that outputs completion scripts:

use clap_complete::{generate, Shell};

#[derive(Subcommand)]
enum Commands {
    /// Generate shell completions
    #[command(hide = true)]
    Completions {
        #[arg(value_enum)]
        shell: Shell,
    },
}

Then document usage in README:

# Bash
syld completions bash > ~/.local/share/bash-completion/completions/syld

# Zsh
syld completions zsh > ~/.local/share/zsh/site-functions/_syld

# Fish
syld completions fish > ~/.config/fish/completions/syld.fish

Priority

Low — quality-of-life improvement for power users