feat(code): add sunbeam-proto crate with gRPC service definition

shared protobuf definitions for the sunbeam code agent:
- CodeAgent service with bidirectional Session streaming
- ClientMessage: StartSession, UserInput, ToolResult, ToolApproval
- ServerMessage: TextDelta, ToolCall, ApprovalNeeded, Status
- ToolDef for client-side tool registration

this is the transport layer between `sunbeam code` (TUI client)
and Sol (server-side agent loop). next: JWT middleware for OIDC
auth, Sol gRPC server stub, CLI subcommand stub.

sunbeam-proto/build.rs

@@ -0,0 +1,4 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tonic_build::compile_protos("proto/code.proto")?;
+    Ok(())
+}