feat: add completion subcommand for bash and zsh shell completion

[dwmkerr]

Summary

• Add shell completion scripts for bash, zsh, and fish in scripts/completions/
• Add generate-completions.sh wrapper that outputs the appropriate script for each shell
• Covers all subcommands (auth, mcp, plugin, install, doctor, setup-token, update), nested subcommands (e.g. mcp add, plugin marketplace list), global flags, and option value completions

Installation

# Bash — add to ~/.bashrc:
eval "$(claude completion bash)"

# Zsh — add to ~/.zshrc:
eval "$(claude completion zsh)"

# Fish — save to completions directory:
claude completion fish > ~/.config/fish/completions/claude.fish

UX pattern

The claude completion <shell> pattern follows established CLI conventions:

• GitHub CLI: gh completion -s bash
• pnpm: pnpm completion bash
• volta: volta completions bash

The internal CLI already has completion infrastructure (shell detection, cache file management at ~/.claude/completion.*, and a registered completion subcommand) — these scripts provide the actual completion definitions that the existing plumbing can serve.

Validation

All scripts pass syntax checks:

• bash -n claude.bash — valid
• zsh -n claude.zsh — valid
• Fish syntax follows the same patterns used by the fish community

Closes #7738