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>

plugins/ai_test/plugin.yaml

@@ -28,4 +28,4 @@ exports:
   - name: run_checks
     description: Run end-to-end Agentsd syscall checks for AI verification
 
-entry: python ai_test_plugin.py
+entry: cargo run -p agentsd-plugin-ai-test