[FEATURE] Add shell completion support (bash/zsh/fish) for claude CLI

Feature Request

Add a built-in shell completion generator for the claude CLI, similar to how other CLI tools handle this:

• kubectl completion zsh
• gh completion -s zsh
• helm completion bash

Proposed command

claude completion bash
claude completion zsh
claude completion fish

Usage

# zsh
source <(claude completion zsh)

# bash
source <(claude completion bash)

Why

Claude Code has a growing number of subcommands (agents, auth, mcp, plugin, project, install, ultrareview, etc.) and flags (--model, --effort, --print, --worktree, --mcp-config, etc.). Tab completion would make the CLI significantly more discoverable and faster to use, especially for less frequently used options.

Environment

• Platform: macOS (darwin)
• Version: 2.1.138