discover a camera when none is configured

CAPTURE_PATH defaulted to index 0, which is a coin flip on a machine with one
camera. A UVC camera exposes a second device node for its metadata stream --
/dev/video1 beside /dev/video0 -- and that node opens happily and never yields
an image, so the lowest device number is wrong about as often as it is right.

With neither CAMERA_ID nor CAPTURE_PATH set, the node now opens each
/dev/video* in numeric order and keeps the first that actually delivers a
frame. Reading one is the test, because opening is not evidence.

Discovery finding nothing falls back to index 0 rather than raising, which
keeps the failure in one place: the first read, where CI publishes a
placeholder and a normal run raises with a message naming the device.

CAMERA_ID is still worth preferring on a robot. Neither an index nor discovery
order is stable across reboots or across which port a camera is in; a by-id
name is.

dora-node.yml

@@ -42,8 +42,11 @@ env:
       which USB port the camera is in, unlike an index, so it takes precedence
       over CAPTURE_PATH. Linux only.
   CAPTURE_PATH:
-    default: "0"
-    description: Device path such as /dev/video1, or an index. A numeric string is read as an index.
+    description: >-
+      Device path such as /dev/video1, or an index. A numeric string is read as
+      an index. When unset, the first /dev/video* that actually delivers a frame
+      is used -- a UVC camera also exposes a metadata node that opens and never
+      captures, so the lowest device number is not a safe default.
   IMAGE_WIDTH:
     type: int
     description: Requested capture width; frames are also resized to it. Defaults to the camera's native width.