Add GitHub Actions CI for lint and tests

[hemna]

Summary

• Add repository-wide GitHub Actions checks for linting and unit tests.
• Use Python 3.12.13, matching the deployed exporter image.
• Run lint and tests as separate jobs on pull requests targeting master and pushes to master.

Verification

• Workflow YAML parses locally.
• The workflow runs flake8 . and stestr run for the full repository.

The workflow is based directly on master so it can be reviewed and merged independently.