Dynamic model routing for DSH subagents

Key features

• The model selects only the LLM route: provider must name a registered DSH LLM adapter route and model a model id on it.
• A continuable child is a durable conversation: send_message (official control tool) delivers later FIFO turns, list_agents lists it, interrupt_agent interrupts it — all through ctx.subagents authority paths.
• Each loaded router instance needs a unique toolName.
• allowedProviders is executor-level enforcement, not a prompt hint.
• provider/model validity is ultimately resolved by the DSH LLM/Agent resolution at the child's first request (no listModels() hard whitelist, preserving dynamic model routes).