Refactor dependency groups and scope CI installs

[gwdio]

Refactors dependency groups in pyproject.toml to ensure that github workflow runs import the minimum amount of packages.

Groups have been updated to the following:

• briefcase — provides cookiecutter; included by test and dev
• pre-commit — used by the pre-commit CI job (--group pre-commit)
• tox — used by the unit tests CI job (--group tox)
• test — full test suite deps; includes briefcase
• dev — local development convenience; includes all of the above

Afterwards, CI has been updated to use the new dependency groups.

This ensures that CI runs import dependency groups logically and do not overimport.

Refs #236

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Claude Sonnet 4.6

[kattni]

Thanks for the PR! One suggestion in-line.

[gwdio]

Should we also update requirements.txt to remove black then?

[freakboy3742]

👍 No need to clean up requirements.txt - it's only going to be there for another couple of hours.