feat: implement skeleton phase

src/beaver_gateway/agents/claude.py

@@ -0,0 +1,17 @@
+"""Claude Code agent definition.
+
+Notice the absence of a ``streaming`` field — claude-code does not emit
+token-level deltas, and that fact is encoded in the type, not in a
+runtime branch.
+"""
+
+from __future__ import annotations
+
+from beaver_gateway.agents.base import BaseAgent
+
+
+class ClaudeAgent(BaseAgent):
+    """Agent backed by ``claude-code-api``."""
+
+    cwd: str
+    available_native_tools: tuple[str, ...] = ()