chore: remove requirements.txt in favour of pyproject.toml

[mwbrooks]

Type of change

• New feature
• Bug fix
• Documentation

Summary

This pull request removes requirements.txt so pyproject.toml is the single source of dependencies. Developers install each implementation and its dev tools with one command: pip install -e ".[dev]".

• Copies the newer pins from requirements.txt into pyproject.toml first, because Dependabot had only been updating requirements.txt: aiohttp 3.14.5 and pydantic-ai[openai,anthropic] 2.55.0
• Installs with pip install -e ".[dev]" in CI and each README
• Matches bolt-python-assistant-template#156
• Closes chore: prefer pyproject.toml over requirements.txt #25, an earlier draft of the same change
• Leaves the /vendor-bolt and /unvendor-bolt commands and the maintainers guide alone. They still describe vendoring the wheel through requirements.txt, and need a follow-up if we vendor again
• Verified each implementation in a fresh Python 3.14 virtual environment: the install, ruff check, ruff format --check, and pytest (7 passed each) all 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]

note: This version was behind requirements.txt. Dependabot bumps each file in separate PRs and some of the pyproject.toml bumps never landed. Removing requirements.txt leaves one file to update, which stops the drift.

[mwbrooks]

note: This version was behind requirements.txt. Dependabot bumps each file in separate PRs and some of the pyproject.toml bumps never landed. Removing requirements.txt leaves one file to update, which stops the drift. The anthropic extra matches requirements.txt.

[mwbrooks]

@zimeg Thanks again x 7 for the review 🙇🏻