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/constructors.go

@@ -16,7 +16,7 @@ func OpenAI(apiKey string, opts ...ClientOption) *Client {
 	for _, opt := range opts {
 		opt(cfg)
 	}
-	return newClient(openaiProvider.New(apiKey, cfg.baseURL))
+	return NewClient(openaiProvider.New(apiKey, cfg.baseURL))
 }
 
 // Anthropic creates an Anthropic client.
@@ -30,7 +30,7 @@ func Anthropic(apiKey string, opts ...ClientOption) *Client {
 		opt(cfg)
 	}
 	_ = cfg // Anthropic doesn't support custom base URL in the SDK
-	return newClient(anthProvider.New(apiKey))
+	return NewClient(anthProvider.New(apiKey))
 }
 
 // Google creates a Google (Gemini) client.
@@ -44,5 +44,5 @@ func Google(apiKey string, opts ...ClientOption) *Client {
 		opt(cfg)
 	}
 	_ = cfg // Google doesn't support custom base URL in the SDK
-	return newClient(googleProvider.New(apiKey))
+	return NewClient(googleProvider.New(apiKey))
 }