feat: add shell completion support for bash, zsh, and fish

[mvanhorn]

Summary

Adds a parallel-cli completion command group for shell tab-completion:

• parallel-cli completion show [--shell bash|zsh|fish] - Print the completion script to stdout
• parallel-cli completion install [--shell bash|zsh|fish] - Auto-install to your shell config file

Uses Click's built-in shell completion system, so completions stay in sync as commands are added. Shell is auto-detected from $SHELL with --shell override.

Usage

# Auto-install for your current shell
parallel-cli completion install

# Or manually add to your config
parallel-cli completion show --shell zsh >> ~/.zshrc

Details

• Supports bash, zsh, and fish
• Install is idempotent (guard comment prevents duplicates)
• Standalone binary mode (PyInstaller) is detected and rejected gracefully with a helpful message - completions require the pip-installed version
• 8 new tests covering all shells, auto-detection, idempotency, and standalone rejection

Research & Context

Project Context

• The CLI has a deep command tree (search, extract, research, enrich, findall, monitor) with many subcommands and options
• Shell completions are table-stakes UX for CLI tools - every mature CLI ships them
• Click 8.1+ has built-in completion support that just needs to be exposed

Competitive Analysis

• Most Python CLI tools using Click ship completion support
• Typer (which wraps Click) auto-generates completion install commands
• This brings parallel-cli in line with CLI UX expectations

This contribution was developed with AI assistance (Claude Code).

[mvanhorn]

thank you!