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>

README.md

@@ -71,14 +71,14 @@ agentsd  ← 对标 Linux 内核 + systemd(PID 1)
 
 ## 当前状态
 
-阶段 1 已完成:内核骨架可运行,9 个 syscall gRPC service + PluginBridge 已注册,Python 测试插件验证全链路通过。默认运行数据写入程序所在目录的 `data/` 文件夹。`plugins/ai_test` 提供手动 AI 验证插件,默认不开启/不自动加载。Release 二进制约 6.1MB。
+阶段 1 已完成:内核骨架可运行,9 个 syscall gRPC service + PluginBridge 已注册,测试插件验证全链路通过。默认运行数据写入程序所在目录的 `data/` 文件夹。`plugins/ai_test` 提供手动 AI 验证插件,默认不开启/不自动加载。Release 二进制约 6.1MB。
 
 ## 手动验证
 
 ```bash
 cargo build --release
 RUST_LOG=agentsd=info ./target/release/agentsd.exe
-python plugins/ai_test/ai_test_plugin.py
+cargo run -p agentsd-plugin-ai-test
 ```
 
 `plugins/ai_test/plugin.yaml` 中设置了 `enabled: false` 和 `autoload: false`,未来实现插件自动加载时应默认跳过它。