The app holds each call of the page as a self-managed Telecom call
(transactional API, foreground service), so a headset's AT+CHUP reaches
it. Page and app talk over the Custom Tabs postMessage channel: the page
reports started/ended, the app sends hangup. assetlinks.json grants the
app use_as_origin for that.

/?listen, which the Android app opens, matched no static file and got 404.

The Android app now handles VOICE_COMMAND (HFP AT+BVRA from headsets),
shows over the lock screen and asks to unlock before opening the page.
It opens /?listen, which starts the call without pressing Talk.

.github/workflows/assist-android.yml builds packages.hermes-voice-android
on master pushes that touch it (and on demand) with the run number as
versionCode and the short commit as versionName, signs it with
hermes-voice-android-sign from the ANDROID_KEYSTORE_B64 and
ANDROID_KEYSTORE_PASSWORD secrets (the clan var
hermes-voice-android-signing of coaxmetal) and uploads it as assist.apk
of the rolling release assist-android.

The package takes versionCode and versionName (default 1), so
`nix build .#hermes-voice-android` stays pure; CI overrides them through
--impure --expr. The unsigned APK is also a check, so PR CI builds it.