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>
