chore: replace requirements.txt with pyproject.toml dependencies

[mwbrooks]

Type of change

• New feature
• Bug fix
• Documentation

Summary

This pull request makes pyproject.toml the single source of dependencies, so developers install the app and its dev tools with one command: pip install -e ".[dev]".

Without a [project] section, pip install -e . returns an error. The Slack CLI runs that same command when it finds a pyproject.toml, so this also covers projects created with slack create.

• Moves the pinned versions from requirements.txt into dependencies and a dev extra for pytest and ruff
• Limits setuptools package discovery to ai, listeners, and state_store, so the editable install works with the flat project layout
• Installs with pip install -e ".[dev]" in CI and the README
• Matches bolt-python-starter-template#71 and bolt-python-custom-step-template#69
• Closes chore: prefer pyproject.toml over requirements.txt #121, an earlier draft of the same change
• Verified in a fresh Python 3.14 virtual environment: the install, ruff check, and ruff format --check succeed

Requirements

• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct

[mwbrooks]

@zimeg Thanks again x 6 for the review! 🙇🏻