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

.dockerignore

@@ -0,0 +1,15 @@
+.git
+.github
+.gcp
+bundle
+evals
+integration-tests
+docs
+packages/cli
+packages/vscode-ide-companion
+packages/test-utils
+**/*.test.ts
+**/*.test.js
+**/src/**/*.ts
+!packages/a2a-server/dist/**
+!packages/core/dist/**