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

examples/source.py

@@ -0,0 +1,73 @@
+"""Synthetic source node: this repository's stand-in for a camera and a detector.
+
+Emits an animated frame, the box around the square moving in it, and a caption --
+the three inputs dora-plot draws -- so the example dataflow needs no camera, no
+detector and no network.
+"""
+
+import os
+
+import numpy as np
+import pyarrow as pa
+from dora import Node
+
+WIDTH = int(os.getenv("SOURCE_WIDTH", "640"))
+HEIGHT = int(os.getenv("SOURCE_HEIGHT", "480"))
+# 0 runs until the viewer quits; a positive count is what lets CI run the
+# dataflow to completion instead of killing it on a timeout.
+FRAMES = int(os.getenv("SOURCE_FRAMES", "0"))
+
+BOX = 96
+SPEED = 6
+CAPTION = "dora-plot synthetic source"
+
+
+def main() -> None:
+    """Send one image, bbox and text per tick."""
+    node = Node("source")
+    sent = 0
+
+    for event in node:
+        if event["type"] != "INPUT":
+            continue
+
+        frame, corners = render(sent)
+        node.send_output(
+            "image",
+            pa.array(frame.ravel()),
+            {"width": WIDTH, "height": HEIGHT, "encoding": "bgr8"},
+        )
+        node.send_output(
+            "bbox",
+            pa.array([{
+                "bbox": corners,
+                "conf": np.array([0.99]),
+                "labels": np.array(["square"]),
+            }]),
+            {"format": "xyxy"},
+        )
+        node.send_output("text", pa.array([CAPTION]))
+
+        sent += 1
+        if FRAMES and sent >= FRAMES:
+            break
+
+
+def render(index: int) -> tuple[np.ndarray, np.ndarray]:
+    """Draw frame `index` and return it with the xyxy corners of the square in it."""
+    frame = np.zeros((HEIGHT, WIDTH, 3), dtype=np.uint8)
+    frame[:, :, 0] = np.linspace(40, 140, HEIGHT, dtype=np.uint8)[:, None]
+
+    travel = WIDTH - BOX
+    # A triangle wave over `index`: the square starts at the left edge, runs to
+    # the right edge, and comes back.
+    left = travel - abs((index * SPEED) % (2 * travel) - travel)
+    top = (HEIGHT - BOX) // 2
+    frame[top : top + BOX, left : left + BOX] = (60, 200, 255)
+
+    corners = np.array([left, top, left + BOX, top + BOX], dtype=np.float64)
+    return frame, corners
+
+
+if __name__ == "__main__":
+    main()