add an in-repo synthetic source so the example needs no network

The example dataflow pulled opencv-video-capture and pyarrow-sender from
dora-hub, needed a webcam, and left `bbox` unwired because every bbox source
is a detector. examples/source.py emits an animated frame, the box around the
square in it, and a caption, so the dataflow exercises all three inputs with
nothing outside this checkout.

SOURCE_FRAMES bounds the run, which lets CI run the dataflow headless rather
than only validate it. Drops the dora-zed and dora-yolo wirings from the
README and trims AGENTS.md to what the code and README do not already say.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018WLVZCFFcCH1EpTXGfS9aD

README.md

@@ -27,44 +27,35 @@ nodes:
 Run the dataflow with `--uv` (`dora run dataflow.yml --uv`) so the node gets its own
 managed environment.
 
+Wire `image` from any node publishing one of the encodings below, and `bbox` from any
+detector publishing the struct below. Both inputs are optional: an `image` alone is a
+plain viewer.
+
 ### Runnable example
 
-[`dataflow.yml`](dataflow.yml) wires a webcam and a fixed caption from dora-hub into
-the viewer:
+[`dataflow.yml`](dataflow.yml) is self-contained. [`examples/source.py`](examples/source.py)
+is a synthetic source node that emits an animated frame, the box around the square
+moving in it, and a caption — the three inputs this node draws — so the example needs
+no camera, no detector and no network:
 
 ```bash
 dora run dataflow.yml --uv
 ```
 
-It builds the node from this checkout (`path:` + `build:`) rather than from a tag,
+A window opens with a square bouncing across a gradient, boxed and labelled. Press `q`
+to quit.
+
+It builds both nodes from this checkout (`path:` + `build:`) rather than from a tag,
 which is what lets `dora validate` pick up the manifest described below.
 
-`bbox` is left unwired there, because every bbox source is a detector and pulling one
-in would put a model download in front of a webcam preview. To see boxes, add
-[`dora-yolo`](https://github.com/dora-rs/dora-hub/tree/main/node-hub/dora-yolo) between
-the camera and the viewer:
+Set `SOURCE_FRAMES` to stop the source after a fixed number of frames instead of running
+until you quit; that plus `CI=true` is how the same dataflow runs headless as a test:
 
-```yaml
-  - id: detector
-    git: https://github.com/dora-rs/dora-hub.git
-    build: pip install ./node-hub/dora-yolo
-    path: dora-yolo
-    inputs:
-      image: camera/image
-    outputs:
-      - bbox
-
-  - id: plot
-    path: dora-plot
-    build: pip install .
-    inputs:
-      image: camera/image
-      bbox: detector/bbox
+```bash
+CI=true SOURCE_FRAMES=30 dora run dataflow.yml --uv
 ```
 
-To display a [dora-zed](https://gitea.liria.cl/liria/dora-zed) camera instead, wire
-`image: zed-camera/image_bgr` — that node publishes `bgr8` with the `width`, `height`
-and `encoding` metadata this one expects.
+`SOURCE_WIDTH` and `SOURCE_HEIGHT` (default 640x480) set the frame size.
 
 ### Node manifest
 
@@ -160,5 +151,6 @@ display to show the window on (or `CI=true` to run without one).
 uv run pytest
 ```
 
-The tests cover decoding, the bbox format conversion and the headless draw path, so they
-need neither a camera, a dataflow nor a display.
+Unit tests over decoding, the bbox format conversion and the headless draw path; they
+need neither a camera, a dataflow nor a display. The headless dataflow run above covers
+the rest — the manifest, the wiring and the event loop.