feature: Added GitHub Actions workflows

[Nacchofer31]

• Added GitHub Actions workflow split into 6 files: pr-checks.yml (orchestrator), build.yml, spotless.yml, unit-tests.yml, instrumented-tests.yml, and coverage.yml
• Workflow triggers on PRs to master and on direct pushes/merges to master
• Jobs run in parallel where possible: build and spotless run independently, unit-tests and instrumented-tests run in parallel after build, and coverage runs after both
  test jobs complete
• Instrumented tests run on an Android API 29 emulator using reactivecircus/android-emulator-runner with KVM enabled - Jacoco execution data from both test jobs is uploaded as artifacts and merged in the coverage job to generate a unified report, which is then uploaded to Codecov
• Spotless automatically applies formatting and commits any changes back to the PR branch