feat(plugin-runtime): add plugin isolation IPC infrastructure

- Protocol layer: Envelope model with Pydantic schema, MsgPack/JSON codecs, unified error codes
- Transport layer: cross-platform IPC abstraction with 4-byte length-prefixed framing (UDS + TCP fallback)
- Host: RPC server, policy engine, circuit breaker, capability service, supervisor with hot-reload
- Runner: RPC client, plugin loader, process entry point
- Tests: 16 passing tests covering protocol, transport, host, and E2E handshake

src/plugin_runtime/runner/plugin_loader.py

@@ -0,0 +1,127 @@
+"""插件加载器
+
+在 Runner 进程中负责发现和加载插件。
+插件通过 SDK 编写，不再 import src.*。
+"""
+
+from typing import Any
+
+import importlib
+import importlib.util
+import json
+import logging
+import os
+import sys
+
+logger = logging.getLogger("plugin_runtime.runner.plugin_loader")
+
+
+class PluginMeta:
+    """加载后的插件元数据"""
+
+    def __init__(
+        self,
+        plugin_id: str,
+        plugin_dir: str,
+        plugin_instance: Any,
+        manifest: dict[str, Any],
+    ):
+        self.plugin_id = plugin_id
+        self.plugin_dir = plugin_dir
+        self.instance = plugin_instance
+        self.manifest = manifest
+        self.version = manifest.get("version", "1.0.0")
+        self.capabilities_required = manifest.get("capabilities", [])
+
+
+class PluginLoader:
+    """插件加载器
+
+    扫描插件目录，加载符合 SDK 规范的插件。
+    每个插件目录须包含：
+    - _manifest.json: 插件元数据
+    - plugin.py: 插件入口模块（导出 create_plugin 工厂函数）
+    """
+
+    def __init__(self):
+        self._loaded_plugins: dict[str, PluginMeta] = {}
+
+    def discover_and_load(self, plugin_dirs: list[str]) -> list[PluginMeta]:
+        """扫描多个目录并加载所有插件
+
+        Args:
+            plugin_dirs: 插件目录列表
+
+        Returns:
+            成功加载的插件元数据列表
+        """
+        results = []
+        for base_dir in plugin_dirs:
+            if not os.path.isdir(base_dir):
+                logger.warning(f"插件目录不存在: {base_dir}")
+                continue
+
+            for entry in os.listdir(base_dir):
+                plugin_dir = os.path.join(base_dir, entry)
+                if not os.path.isdir(plugin_dir):
+                    continue
+
+                manifest_path = os.path.join(plugin_dir, "_manifest.json")
+                plugin_path = os.path.join(plugin_dir, "plugin.py")
+
+                if not os.path.exists(manifest_path) or not os.path.exists(plugin_path):
+                    continue
+
+                try:
+                    meta = self._load_single_plugin(plugin_dir, manifest_path, plugin_path)
+                    if meta:
+                        self._loaded_plugins[meta.plugin_id] = meta
+                        results.append(meta)
+                except Exception as e:
+                    logger.error(f"加载插件失败 [{plugin_dir}]: {e}", exc_info=True)
+
+        return results
+
+    def get_plugin(self, plugin_id: str) -> PluginMeta | None:
+        """获取已加载的插件"""
+        return self._loaded_plugins.get(plugin_id)
+
+    def list_plugins(self) -> list[str]:
+        """列出所有已加载的插件 ID"""
+        return list(self._loaded_plugins.keys())
+
+    def _load_single_plugin(self, plugin_dir: str, manifest_path: str, plugin_path: str) -> PluginMeta | None:
+        """加载单个插件"""
+        # 1. 读取 manifest
+        with open(manifest_path, "r", encoding="utf-8") as f:
+            manifest = json.load(f)
+
+        plugin_id = os.path.basename(plugin_dir)
+
+        # 2. 动态导入插件模块
+        module_name = f"_maibot_plugin_{plugin_id}"
+        spec = importlib.util.spec_from_file_location(module_name, plugin_path)
+        if spec is None or spec.loader is None:
+            logger.error(f"无法创建模块 spec: {plugin_path}")
+            return None
+
+        module = importlib.util.module_from_spec(spec)
+        sys.modules[module_name] = module
+        spec.loader.exec_module(module)
+
+        # 3. 调用工厂函数创建插件实例
+        create_plugin = getattr(module, "create_plugin", None)
+        if create_plugin is None:
+            logger.error(f"插件 {plugin_id} 缺少 create_plugin 工厂函数")
+            return None
+
+        instance = create_plugin()
+
+        logger.info(f"插件 {plugin_id} v{manifest.get('version', '?')} 加载成功")
+
+        return PluginMeta(
+            plugin_id=plugin_id,
+            plugin_dir=plugin_dir,
+            plugin_instance=instance,
+            manifest=manifest,
+        )