feat(cli): enable shell completion for bash/zsh/fish

[ulises-jeremias]

Summary

• Enables Typer add_completion=True so --install-completion / --show-completion are available
• Documents bash/zsh/fish completion setup in package and root READMEs
• Extends help-flag coverage to assert completion options are advertised

Closes#166

Test plan

• uv run pytest packages/create-awesome-python-app/tests/test_flags.py packages/create-awesome-python-app/tests/test_cli.py -q
• uv run ruff check packages/create-awesome-python-app/src/create_awesome_python_app/cli.py packages/create-awesome-python-app/tests/test_flags.py

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