Tool and tool decorator

@tool builds a Tool from a typed callable. The signature becomes an object JSON schema; the docstring becomes the model description. A bare Context parameter is injected.

Tool.invoke(args, ctx) validates input and executes the callable. Runtime starts accepted tool calls in parallel, offloading synchronous functions to threads, and returns results to the provider in call order.

Framework actor tools are reserved when applicable: spawn(agent_name, input, name=None) and status(agent_id) on declarations with children, send(agent_id, input) across a direct parent-child edge, and child-only finish(result). A spawn name is an optional trimmed display label and does not change actor identity. Human asks remain root-only.