Migrate test plugins to Rust and async-ify syscall modules

- Replace Python test plugins (echo, ai_test, claudecode) with Rust
  implementations driven by the plugin SDK
- Convert fs/network/process/timer syscall modules to async/await
- Change Network.OpenConn to return stream ConnEvent (conn_id handed
  to client in first "open" frame for Send/Close routing)
- Bind timer callbacks to plugin identity; stream_call subscriptions
  are identity-checked
- Update docs and regenerate Python SDK protobuf stubs

Verified: cargo build/test/clippy clean; echo + ai_test full syscall
suites pass; hermes & claudecode register and heartbeat; cross-plugin
bridge.call routing and auth denial verified end-to-end.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

proto/agentsd.proto

@@ -93,7 +93,7 @@ message MemoryResponse { bool ok = 1; string error = 2; }
 // ========== Network ==========
 service Network {
   rpc Request(HttpRequest) returns (HttpResponse);
-  rpc OpenConn(ConnectRequest) returns (stream DataChunk);
+  rpc OpenConn(ConnectRequest) returns (stream ConnEvent);
   rpc Listen(ListenRequest) returns (stream ConnEvent);
   rpc Send(SendRequest) returns (NetResponse);
   rpc Close(CloseRequest) returns (NetResponse);