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

.gitea/workflows/validate.yml

@@ -1,5 +1,6 @@
-# Two gates, neither of which needs a camera or a display: the dataflow still
-# matches the typed contract in dora-node.yml, and the node's own tests pass.
+# Two gates, neither of which needs a camera or a display: the example dataflow
+# still matches the typed contract in dora-node.yml and still runs end to end,
+# and the node's own unit tests pass.
 name: validate
 
 # A pull request would otherwise run this twice on the same commit: once for
@@ -14,11 +15,22 @@ env:
   DORA_TARGET: x86_64-unknown-linux-gnu
 
 jobs:
-  validate:
+  dataflow:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
 
+      # The opencv-python wheel links against libGL at import time, so cv2
+      # cannot even be imported on the bare runner image.
+      - name: Install the OpenCV shared libraries
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends libgl1 libglib2.0-0
+
+      - uses: astral-sh/setup-uv@v5
+        with:
+          enable-cache: true
+
       - name: Install the dora CLI
         run: |
           curl -fsSL -o dora-cli.tar.gz \
@@ -36,13 +48,19 @@ jobs:
       - name: Validate the dataflow
         run: dora validate dataflow.yml
 
+      # CI=true is already set by the runner, which keeps the viewer from trying
+      # to open a window; SOURCE_FRAMES bounds the source so the run ends on its
+      # own. Everything the dataflow needs is in the checkout.
+      - name: Run the dataflow headless
+        env:
+          SOURCE_FRAMES: 30
+        run: dora run dataflow.yml --uv
+
   test:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
 
-      # The opencv-python wheel links against libGL at import time, so cv2
-      # cannot even be imported on the bare runner image.
       - name: Install the OpenCV shared libraries
         run: |
           sudo apt-get update
@@ -59,6 +77,4 @@ jobs:
 
       - run: uv run ruff check .
 
-      # CI=true is already set by the runner, which is what keeps the node from
-      # trying to open a window on a headless machine.
       - run: uv run pytest -q