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/protocol/errors.py

@@ -0,0 +1,61 @@
+"""RPC 错误码定义
+
+所有 Host 与 Runner 之间的 RPC 通信使用统一的错误码体系。
+"""
+
+from enum import Enum
+
+
+class ErrorCode(str, Enum):
+    """RPC 错误码枚举"""
+
+    # 通用
+    OK = "OK"
+    E_UNKNOWN = "E_UNKNOWN"
+
+    # 协议层
+    E_TIMEOUT = "E_TIMEOUT"
+    E_BAD_PAYLOAD = "E_BAD_PAYLOAD"
+    E_PROTOCOL_MISMATCH = "E_PROTOCOL_MISMATCH"
+
+    # 权限与策略
+    E_UNAUTHORIZED = "E_UNAUTHORIZED"
+    E_METHOD_NOT_ALLOWED = "E_METHOD_NOT_ALLOWED"
+    E_BACKPRESSURE = "E_BACKPRESSURE"
+    E_HOST_OVERLOADED = "E_HOST_OVERLOADED"
+
+    # 插件生命周期
+    E_PLUGIN_CRASHED = "E_PLUGIN_CRASHED"
+    E_PLUGIN_NOT_FOUND = "E_PLUGIN_NOT_FOUND"
+    E_GENERATION_MISMATCH = "E_GENERATION_MISMATCH"
+    E_RELOAD_IN_PROGRESS = "E_RELOAD_IN_PROGRESS"
+
+    # 能力调用
+    E_CAPABILITY_DENIED = "E_CAPABILITY_DENIED"
+    E_CAPABILITY_FAILED = "E_CAPABILITY_FAILED"
+
+
+class RPCError(Exception):
+    """RPC 调用异常"""
+
+    def __init__(self, code: ErrorCode, message: str = "", details: dict | None = None):
+        self.code = code
+        self.message = message or code.value
+        self.details = details or {}
+        super().__init__(f"[{code.value}] {self.message}")
+
+    def to_dict(self) -> dict:
+        return {
+            "code": self.code.value,
+            "message": self.message,
+            "details": self.details,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "RPCError":
+        code = ErrorCode(data.get("code", "E_UNKNOWN"))
+        return cls(
+            code=code,
+            message=data.get("message", ""),
+            details=data.get("details", {}),
+        )