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.

Cargo.toml

@@ -1,3 +1,3 @@
 [workspace]
-members = ["sunbeam-sdk", "sunbeam"]
+members = ["sunbeam-sdk", "sunbeam", "sunbeam-proto"]
 resolver = "3"