Automatically reconnect single ADB head unit

README.md

@@ -151,15 +151,19 @@ Important values:
 - `UPDATE_STATUS_PATH`: update state shared with the systemd update unit
 - `VERSION_FILE`: revision metadata for the active atomic release
 - `ADB_ENABLED`: enables the read-only head-unit collector; defaults to `false`
-- `ADB_SERIAL`: exact device serial used for every ADB command when enabled
+- `ADB_SERIAL`: optional exact serial; when empty, discovery requires exactly one attached device
 - `ADB_PATH` and `ADB_TIMEOUT_MS`: executable path and per-command timeout
 
 ### Read-only ADB setup
 
-ADB integration is optional and disabled by default. Every command is pinned to one
-configured serial, uses a fixed server-side argument list, and has a timeout and 16 KiB
-output limit. It currently reads connection state, selected `getprop` identity fields,
-and `wm size`/`wm density` only.
+ADB integration is optional and disabled by default. It starts the ADB server when the
+companion service boots. With no configured serial it automatically selects exactly one
+attached device and refuses ambiguous multi-device connections; after selection every
+command is serial-pinned. Commands use fixed server-side argument lists with a timeout
+and 16 KiB output limit. The ADB authorization key persists in the companion data
+directory, so an already-authorized car reconnects after Pi reboot or USB reattachment.
+The collector currently reads connection state, selected `getprop` identity fields, and
+`wm size`/`wm density` only.
 
 On a deployed Pi, rerun `install.sh` once so `android-tools-adb` is present and the
 service account receives the standard `plugdev` device-access group. Connect the head
@@ -174,7 +178,8 @@ Accept the authorization prompt on the parked infotainment screen. Set these val
 
 ```text
 ADB_ENABLED=true
-ADB_SERIAL=<exact serial shown by adb devices>
+# Optional when only the MG4 is attached:
+ADB_SERIAL=
 ```
 
 If the device is disconnected, offline, unauthorized, partially readable, or ADB is