feat: implement skeleton phase

src/beaver_gateway/agents/base.py

@@ -0,0 +1,33 @@
+"""Shared agent surface.
+
+``BaseAgent`` is the structural contract every backend-specific agent
+honours. Subclasses add **capabilities as fields** (``ClaudeAgent.cwd``,
+``RaycastAgent.streaming``) — backends dispatch on type, not on a runtime
+matrix.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from pydantic import BaseModel, ConfigDict
+
+
+@dataclass(frozen=True, slots=True)
+class ExposedMcp:
+    """Reference to an ``McpServer`` (by name) exposed to a single agent."""
+
+    name: str
+    tools: tuple[str, ...] | None = None
+
+
+class BaseAgent(BaseModel):
+    """Common fields. Concrete backends subclass and add capabilities."""
+
+    model_config = ConfigDict(frozen=True, arbitrary_types_allowed=True)
+
+    name: str
+    model: str
+    system_prompt: str
+    expose_mcps: tuple[ExposedMcp, ...] = ()
+    accept_client_tools: bool = False