Add GitHub Actions CI workflow for firmware build and test

[flwrenn]

Summary

Add a GitHub Actions workflow that compiles the firmware and runs unit tests on every PR to dev, catching build regressions before merge.

Area

Tooling / CI

Done when

• Workflow triggers on PRs to dev
• Builds firmware for supermini_nrf52840 (compile check)
• Builds and runs native_sim test suite
• Green check / red X reported on PR

Dependencies

Depends on #53 (test infrastructure)

Context

Current CI only handles PR housekeeping (issue closure, branch cleanup). No build verification exists. As algorithm code is added, we need compile and test checks to prevent regressions.