feat: add A2UI extension support and GKE deployment infrastructure

Implement A2UI v0.10 protocol compliance for the a2a-server:
- Add a2ui-extension.ts with constants, Part helpers, extension detection
- Add a2ui-components.ts with standard catalog component builders
- Add a2ui-surface-manager.ts for tool approval, response, thought surfaces
- Integrate A2UI into task.ts and executor.ts
- Add configurable bind host (CODER_AGENT_HOST) for k8s compatibility

Add GKE deployment infrastructure:
- Dockerfile, cloudbuild.yaml, k8s deployment manifest, .dockerignore

packages/a2a-server/Dockerfile

@@ -0,0 +1,28 @@
+# Pre-built production image for a2a-server
+# Used with Cloud Build: npm install + build runs in step 1, then Docker copies artifacts
+FROM docker.io/library/node:20-slim
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+  python3 curl git jq ripgrep ca-certificates \
+  && apt-get clean && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+# Copy everything including pre-installed node_modules and pre-built dist
+COPY package.json package-lock.json ./
+COPY node_modules/ node_modules/
+COPY packages/core/ packages/core/
+COPY packages/a2a-server/ packages/a2a-server/
+
+# Create workspace directory for agent operations
+RUN mkdir -p /workspace && chown -R node:node /workspace
+
+USER node
+
+ENV CODER_AGENT_WORKSPACE_PATH=/workspace
+ENV CODER_AGENT_PORT=8080
+ENV NODE_ENV=production
+
+EXPOSE 8080
+
+CMD ["node", "packages/a2a-server/dist/src/http/server.js"]