initial a2a-pack

Dockerfile

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+# Base image for A2A agents. User agents `FROM` this so the SDK is
+# pre-installed and rebuilds are fast.
+FROM python:3.11-slim
+
+WORKDIR /sdk
+
+COPY pyproject.toml ./
+COPY a2a_pack ./a2a_pack
+
+RUN pip install --no-cache-dir .
+
+# Sanity check
+RUN python -c "import a2a_pack; print('a2a-pack', a2a_pack.__name__, 'ok')"
+
+WORKDIR /app