Maintenance around supported Django, Python and Node package versions

[nickmoreton]

Support Reference

Summary

• raise package requirements to Python >=3.9 and Django >=4.2
• update classifiers for supported Python, Django, and Wagtail versions
• update tox and CI Python matrices for currently supported Django/Python combinations
• refresh Poetry lock metadata for the new Python constraint
• add repository AGENTS.md guidance
• update npm development dependencies to the current installed versions from npm outdated
• migrate ESLint configuration to flat config for ESLint 10
• update the project Node version from 20 to 24
• format Markdown files now covered by Prettier 3

CI compatibility notes

• update flake8 to 7.3.0 so linting works on Python 3.14
• update the flake8 ignore config format for flake8 7's stricter parsing
• adjust tox so test commands run inside each tox environment via python -m pytest
• add explicit tox test dependencies and Django/crispy-forms constraints so each Django factor installs the intended versions
• normalize Django 5.2's aria-invalid="true" output so existing snapshots remain consistent across supported Django versions
• update ESLint from 7.32.0 to 10.2.1 and replace .eslintrc.js with eslint.config.js
• update Prettier from 2.3.2 to 3.8.3, Sass from 1.69.7 to 1.99.0, Vite from 2.9.18 to 8.0.10, and Autoprefixer from 10.4.2 to 10.5.0
• use Node 24 in local nvm setup and GitHub Actions via .nvmrc

Testing

• poetry run flake8 tbxforms tests
• poetry run pytest
• poetry run tox -l
• poetry run tox -e py312-dj42
• poetry run tox -e py312-dj52
• npm --cache /private/tmp/tbxforms-npm-cache outdated
• npm --cache /private/tmp/tbxforms-npm-cache ls autoprefixer eslint prettier sass vite --depth=0
• npm run lint
• npm run build

Includes changes made using AI (Codex)