Add shell completions (bash, zsh, fish)

Generate shell completions for the async-inspect CLI.

Description

Enable tab-completion for async-inspect commands in popular shells.

Goals

• Bash completions
• Zsh completions
• Fish completions
• Installation instructions

Implementation

Use clap's built-in completion generation:

use clap_complete::{generate, Shell};

Installation Instructions to Document

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

# Zsh
async-inspect completions zsh > ~/.zfunc/_async-inspect

# Fish
async-inspect completions fish > ~/.config/fish/completions/async-inspect.fish

Acceptance Criteria

• async-inspect completions <shell> command
• Works for bash, zsh, fish
• Installation docs in README
• Completions for all subcommands and flags