ci: add dependabot automerge workflow

[headless-samber]

Add a GitHub Actions workflow to automatically enable auto-merge (squash) for Dependabot PRs that update github_actions dependencies.

• Triggers only on opened and synchronize pull_request events to avoid unnecessary runs
• Restricted to dependabot[bot] actor
• Only applies to the github_actions package ecosystem

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.14%. Comparing base (b7396dd) to head (fb242ee).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #52   +/-   ##
=======================================
  Coverage   75.14%   75.14%           
=======================================
  Files          13       13           
  Lines         342      342           
=======================================
  Hits          257      257           
  Misses         85       85           

Flag	Coverage Δ
unittests	75.14% <ø> (ø)

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