v0.6.0 — remove the handoffs feature

Breaking change: the conversation-handoffs feature is removed. The package is now purely about durable workflow processes: steps, checkpoints, interrupts, and retries.

Removed: HasHandoffs / HasHandoffTools, the synthetic transfer_to_* tools, AgentWorkflow::resolveAgentFor() / transferConversation(), the ConversationTransferred event, and the agent_conversation_owners table (drop it if you migrated it).

Why: conversation-ownership routing was mechanically independent of the workflow engine — a chat-shaped concern living in a pipeline-shaped package. Removing it keeps one concept per package.

If you were using it: stay on v0.5.1 or vendor the handoff classes from that tag — the feature was self-contained (a contract, a trait, a tool, a listener, a model, and two facade methods) and composes with any laravel/ai app without this package.