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

[HelenGreul]

Summary

Adds a poly completion <shell> command that prints a shell completion script, enabling tab completion for all poly/adk commands, subcommands, and flags.

Motivation

Shell completion is a standard ergonomic feature for any CLI tool — especially one with 12+ subcommands and numerous flags. Without it, users must remember exact command names and flags, slowing down daily use. Every major comparable CLI (Google ADK, AWS AgentCore, gh, kubectl) ships with completion support.

Changes

• Add argcomplete>=3.0.0 dependency (Apache 2.0 — passes license checks)
• Call argcomplete.autocomplete(parser) in main() — this is the hook that makes tab completion work at runtime; it exits immediately when not in a completion context, so there is zero overhead for normal CLI usage
• Add completion subparser with bash, zsh, fish choices and per-shell installation instructions in the help text
• Add AgentStudioCLI.print_completion() classmethod
• Both poly and adk entry points are registered in the generated scripts

Usage

# Bash — add to ~/.bashrc or ~/.bash_profile
eval "$(poly completion bash)"

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

# Fish — add to ~/.config/fish/completions/poly.fish
poly completion fish | source

Test strategy

• Added unit tests for all three shells and invalid shell rejection
• Manual CLI testing (poly <command>)
• Tested against a live Agent Studio project
• N/A (docs, config, or trivial change)

Checklist

• ruff check . and ruff format --check . pass
• pytest passes
• No breaking changes to the poly CLI interface (or migration path documented)
• Commit messages follow conventional commits

[jsanter27]

Hi @HelenGreul , appreciate the nice feature here! I've enabled the Actions workflow for you as a first time contributor to the repo. PR looks good, can you just fix the formatting to pass CI? Should be good after that ✅

[jsanter27]

LGTM, thanks again!

[HelenGreul]

Thanks for the quick review and for enabling the workflow @jsanter27!
Took two fixes - formatting and a missing licenses.json entry for the new argcomplete dependency