feat: implement raycast backend

src/beaver_gateway/mcp/types.py

@@ -7,6 +7,7 @@ discriminated-union members so downstream code can ``match`` on ``kind``.
 from __future__ import annotations
 
 from collections.abc import Callable  # noqa: TC003 — runtime use by pydantic
+from pathlib import Path
 from typing import TYPE_CHECKING, Annotated, Literal
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -27,7 +28,7 @@ class StdioMcp(_BaseMcp):
     kind: Literal["stdio"] = "stdio"
     command: tuple[str, ...]
     env: dict[str, str] | None = None
-    cwd: str | None = None
+    cwd: Path | None = None
 
 
 class HttpMcp(_BaseMcp):
@@ -60,9 +61,14 @@ class McpServer:
         name: str,
         command: Iterable[str],
         env: dict[str, str] | None = None,
-        cwd: str | None = None,
+        cwd: Path | str | None = None,
     ) -> StdioMcp:
-        return StdioMcp(name=name, command=tuple(command), env=env, cwd=cwd)
+        return StdioMcp(
+            name=name,
+            command=tuple(command),
+            env=env,
+            cwd=Path(cwd) if isinstance(cwd, str) else cwd,
+        )
 
     @classmethod
     def http(