Workflow docs: "default to omitting opts.model" makes every bare agent() run on the session model — the most expensive tier

The Workflow script reference (Claude Code 2.1.271) says of `agent()`:

> opts.model overrides the model for this agent call. Default to omitting it — the agent inherits the main-loop model (the resolved session model), which is almost always correct. Only set it when you're highly confident a different tier fits the task; when unsure, omit.

On a premium session model that default is the costliest option available, and the guidance steers authors into it.

What we measured (Windows 11, ground truth read from each subagent transcript's `message.model`, not self-report):

- An 8-cell probe confirmed a bare `agent()` (no `agentType`, no `model`) runs on the session model, and that an `agentType` whose definition pins `model:` in frontmatter **is** honoured inside a workflow.
- Every spawn pays roughly 50–180k cache-read plus 40–140k cache-write tokens of preamble before it does any work. At list prices a 100k + 100k preamble is about 9x more expensive on the top tier than on Haiku.
- Over one working day, 41 bare `agent()` spawns accounted for $643.72 — 54% of all subagent spend. The same token volumes would have cost $170.76 on Sonnet and $85.38 on Haiku.

Suggested replacement text:

> Omit it only when opts.agentType names a definition that pins a model — a definition's frontmatter `model:` is honoured inside a workflow. A bare agent() with neither agentType nor model inherits the session model, and each spawn carries roughly 100k cached input tokens of preamble before it does any work, so on a premium session that default is the most expensive option rather than the safest. When neither applies, name the tier the stage actually needs.

Related, different defect: #88695 (`meta.phases[].model` reads as an override but is display-only).

Start at the Workflow script reference entry for agent(), especially the opts.model and opts.agentType guidance. Update the documentation to distinguish bare calls from definitions with frontmatter model:, retain the related distinction from #88695, and confirm the published text matches the suggested replacement.

Written by the indexing model from the issue text.