feat(a2a-bridge): add gRPC transport

.design/a2a-grpc-transport.md

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+# A2A Bridge: gRPC Transport
+
+**Status:** Implementing
+**Created:** 2026-06-05
+**Related:** [a2a-bridge-design.md](./a2a-bridge-design.md), [a2a-multi-turn-lifecycle.md](./a2a-multi-turn-lifecycle.md)
+
+---
+
+## 1. Problem
+
+The A2A bridge currently only supports JSON-RPC 2.0 over HTTP. The A2A protocol
+specification defines three transport bindings: JSON-RPC, gRPC, and HTTP+JSON/REST.
+The ITK compatibility tests exercise gRPC transport in their multi-hop traversal.
+Without gRPC, the bridge cannot participate in cross-SDK interop scenarios that
+use gRPC transport.
+
+## 2. Design
+
+### Approach: gRPC adapter over existing Bridge logic
+
+The Bridge struct already implements all A2A operations (SendMessage, GetTask,
+ListTasks, CancelTask, push notifications, streaming). The gRPC server is a thin
+adapter that:
+
+1. Receives gRPC requests
+2. Translates protobuf messages to the bridge's internal types
+3. Delegates to the same Bridge methods the JSON-RPC server uses
+4. Translates responses back to protobuf
+
+This avoids duplicating any business logic.
+
+### Proto compilation
+
+Use the official A2A proto from `a2aproject/A2A/specification/a2a.proto` as the
+source. Compile with `protoc` + `protoc-gen-go` + `protoc-gen-go-grpc` to generate
+Go code. Generated code goes in `internal/a2apb/`.
+
+The proto has dependencies on `google/api/annotations.proto` etc. — use buf or
+vendored google API protos.
+
+### Server structure
+
+```
+extras/scion-a2a-bridge/
+  internal/
+    a2apb/           # generated protobuf Go code
+    bridge/
+      grpc_server.go # gRPC adapter implementing A2AServiceServer
+```
+
+### gRPC service methods → Bridge method mapping
+
+| gRPC RPC | Bridge method |
+|---|---|
+| SendMessage | Bridge.SendMessage (blocking or non-blocking based on return_immediately) |
+| SendStreamingMessage | Bridge.SendStreamingMessage |
+| GetTask | Bridge.GetTask |
+| ListTasks | Bridge.ListTasks |
+| CancelTask | Bridge.CancelTask |
+| SubscribeToTask | Bridge.SubscribeToTask |
+| CreateTaskPushNotificationConfig | Bridge.SetPushNotificationConfig |
+| GetTaskPushNotificationConfig | Bridge.GetPushNotificationConfig |
+| ListTaskPushNotificationConfigs | Bridge.GetPushNotificationConfig |
+| DeleteTaskPushNotificationConfig | Bridge.DeletePushNotificationConfig |
+| GetExtendedAgentCard | Bridge.GenerateAgentCard |
+
+### Configuration
+
+Add to config:
+```yaml
+bridge:
+  grpc_listen_address: ":9443"  # separate port for gRPC
+```
+
+### Startup
+
+The main() function starts both HTTP and gRPC servers. gRPC is optional — only
+started if grpc_listen_address is configured.
+
+## 3. Scope
+
+- In: gRPC server adapter, proto compilation, config, startup wiring
+- Out: gRPC client (for making outbound A2A calls), TLS on gRPC (use reverse proxy)
+
+## 4. Testing
+
+- Test: each gRPC RPC maps correctly to Bridge method
+- Test: protobuf ↔ internal type translation
+- Test: streaming RPCs deliver events correctly
+- Test: gRPC server starts and accepts connections