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/llm.go

@@ -12,8 +12,9 @@ type Client struct {
 	middleware []Middleware
 }
 
-// newClient creates a Client backed by the given provider.
-func newClient(p provider.Provider) *Client {
+// NewClient creates a Client backed by the given provider.
+// Use this to integrate custom provider implementations or for testing.
+func NewClient(p provider.Provider) *Client {
 	return &Client{p: p}
 }