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/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "sunbeam-proto"
+version = "0.1.0"
+edition = "2024"
+description = "Shared protobuf definitions for Sunbeam gRPC services"
+
+[dependencies]
+tonic = "0.13"
+prost = "0.13"
+
+[build-dependencies]
+tonic-build = "0.13"