Add sub-agent support as tools

[JVillator0]

Agents can now be returned directly from the tools() method, allowing them to act as sub-agents. Following the industry-standard pattern of treating sub-agents as tools, the parent agent's LLM invokes sub-agents like any other tool, passing a task description and receiving the result text.

class ProjectManager implements Agent, HasTools
{
    use Promptable;

    public function instructions(): string
    {
        return 'You delegate research tasks to your sub-agent.';
    }

    public function tools(): iterable
    {
        return [
            new WebSearch,
            new ResearchAgent,
        ];
    }
}

Sub-agents are full agents, they support attributes, their own tools, structured output, and any other agent capability:

#[Provider('anthropic')]
#[Model('claude-sonnet-4-5')]
#[MaxSteps(5)]
class ResearchAgent implements Agent, HasTools
{
    use Promptable;

    public function name(): string
    {
        return 'research_agent';
    }

    public function description(): string
    {
        return 'Research a topic in depth and return a summary.';
    }

    public function instructions(): string
    {
        return 'You are a research agent. Summarize your findings concisely.';
    }

    public function tools(): iterable
    {
        return [new WebSearch];
    }
}

Sub-agents can optionally define name() and description() methods for better tool identification. If omitted, the class basename and instructions() are used as fallbacks.

Each sub-agent invocation runs with isolated context, no conversation history is shared from the parent, keeping context clean and preventing bloat.

Approach

• AgentTool: adapter class that wraps an Agent as a Tool with a { task: string } schema
• GeneratesText::resolveTools(): automatically wraps any Agent found in tools() into an AgentTool
• Both GeneratesText and StreamsText use resolveTools() instead of reading tools() directly
• Sub-agent errors are caught and returned as tool results instead of killing the parent flow

Ref: #75, #58