v0.3.0 of akashi

Provider-neutral core for the Akashi agent framework: streaming tool-loop agents, typed tools, sealed message/event unions, multi-agent and durable-execution seams. Pure Dart. Changelog excerpt: Multi-agent + durable-execution milestone (additive; one noted sealed-union extension). - Subagent-as-tool— Agent.asTool(...)(extension) turns any agent into a Toola parent can call, running the child fresh with isolated history and tools. - Handoffs— Handoff/handoff(...)expose transfer_to_tools; the loop switches the active agent's model/instructions/tools while preserving history, emitting a new HandoffEvent. API note: HandoffEventis a new subtype of the sealed[...]