GitHub Actions built only on tag pushes, so nothing verified that main
compiles. Adds ci.yml running check, unit tests, clippy and a release
build on Linux, plus a Windows MSYS2 build, with both binaries uploaded
as artifacts.
Two issues would have made that workflow hang or fail:
- Unknown CLI arguments fell through the dispatch match into the global
hotkey loop, so `scythe --version` (a typo, or any unrecognised flag)
silently became a background hotkey daemon that never exited. Unknown
arguments now report the flag and exit 2.
- test_audio_probe asserts that a system audio device opens and emits
samples. Headless runners have no audio server, so the suite could
never pass in CI. The test is now opt-in via SCYTHE_HW_TESTS=1 and
still runs on a real desktop session.
