chore: let Dependabot update GitHub Actions weekly

[fschmtt]

Adds the github-actions package ecosystem to .github/dependabot.yml so Dependabot checks the actions used in our workflows once a week and opens PRs for new versions.

Previously only Composer dependencies were tracked, which meant action versions had to be bumped manually (see #180).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.18%. Comparing base (b34c84d) to head (bb6b445).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #181   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files          32       32           
  Lines         972      972           
=======================================
  Hits          896      896           
  Misses         76       76           

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