Add agent sub-package for composable LLM agents

Introduces v2/agent with a minimal API: Agent, New(), Run(), and AsTool().
Agents wrap a model + system prompt + tools. AsTool() turns an agent into
a llm.Tool, enabling parent agents to delegate to sub-agents through the
normal tool-call loop — no channels, pools, or orchestration needed.

Also exports NewClient(provider.Provider) for custom provider integration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/model_test.go

@@ -117,7 +117,7 @@ func TestModel_Complete_WithTools(t *testing.T) {
 
 func TestClient_Model(t *testing.T) {
 	mp := newMockProvider(provider.Response{Text: "hi"})
-	client := newClient(mp)
+	client := NewClient(mp)
 	model := client.Model("test-model")
 
 	resp, err := model.Complete(context.Background(), []Message{UserMessage("test")})
@@ -144,7 +144,7 @@ func TestClient_WithMiddleware(t *testing.T) {
 	}
 
 	mp := newMockProvider(provider.Response{Text: "ok"})
-	client := newClient(mp).WithMiddleware(mw)
+	client := NewClient(mp).WithMiddleware(mw)
 	model := client.Model("test-model")
 
 	_, err := model.Complete(context.Background(), []Message{UserMessage("test")})