feat(task): allow model parameter in task() #29984

Problem

task() inherits the parent model. Cannot use a cheap model (Qwen3-4B) for INVESTIGADOR while the parent uses DeepSeek V4.

Proposed API

task({
  subagent_type: "investigador",
  prompt: "...",
  model: "qwen3-4b"   // optional, overrides parent model
})

Implementation notes

From packages/opencode/src/tool/task.ts:

• BaseParameterFields has: description, prompt, subagent_type, task_id, command — no model parameter
• Model is currently derived from next.model ?? { modelID: msg.info.modelID, providerID: msg.info.providerID }
• Adding an optional model string parameter that overrides this derivation would enable the use case

References

• AGENTS.md §Router de documentación → Activar INVESTIGADOR
• ~/.config/opencode/agents/investigador.md