feat: ship production-grade reference applications

[codeforester]

Summary

• add four independently installable reference applications: minimal, nested/plugin Click, Typer, and automation/observability
• add per-application packaging, tests, completion, release, and operational guidance
• add installed-wheel CI coverage and repository validators for reference examples
• include the reference catalog in documentation and source distributions

Validation

• python3 scripts/validate_examples.py
• python3 scripts/validate_docs.py
• .venv/bin/python -m pytest -q
• all four reference-app test suites via PYTHONPATH=lib/python:examples/<app>/src
• ruff check and ruff format --check
• ./tests/validate.sh

Closes #71