feat: implement raycast backend

src/beaver_gateway/agents/claude.py

@@ -7,11 +7,21 @@ runtime branch.
 
 from __future__ import annotations
 
+from pathlib import Path  # noqa: TC003 — runtime use by pydantic
+
 from beaver_gateway.agents.base import BaseAgent
 
 
 class ClaudeAgent(BaseAgent):
-    """Agent backed by ``claude-code-api``."""
+    """Agent backed by ``claude-code-api``.
 
-    cwd: str
+    ``cwd`` is the working directory the claude CLI is spawned in;
+    typed as ``Path`` (Pydantic coerces strings) since claude-code-api
+    accepts ``str | os.PathLike[str]``. ``available_native_tools`` stays
+    ``tuple[str, ...]`` because Claude Code's tool set is a moving target
+    (Bash/Read/Edit/Write/WebSearch/etc. — versions add and rename), so
+    pinning it as a ``Literal`` would rot the type each release.
+    """
+
+    cwd: Path
     available_native_tools: tuple[str, ...] = ()