Group dependabot PRs together

[alexdewar]

Description

As we've done elsewhere, group dependabot PRs to reduce the number of PRs created. I've made two groups, one for Python deps and another for GitHub Actions.

Closes #58.

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass (eg. python -m pytest)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future.
  (Indicate issue here: # (issue))

[Copilot]

Pull request overview

Updates the repository’s Dependabot configuration to group dependency update pull requests, reducing PR noise while keeping weekly update cadence.

Changes:

• Group all uv (Python) dependency updates into a single Dependabot PR.
• Group all github-actions updates into a single Dependabot PR.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.61%. Comparing base (c0a7f0a) to head (8dbfb74).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #60   +/-   ##
=======================================
  Coverage   47.61%   47.61%           
=======================================
  Files           3        3           
  Lines          42       42           
  Branches        4        4           
=======================================
  Hits           20       20           
  Misses         22       22           

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