docs(registry): group each role's model and effort anchors in variables examples

[lukeaus]

Replaces the flat sibling effort anchors with one grouped anchor pair per role, so a role's model and its reasoning effort sit together:

variables:
  worker:
    model: &worker_model
      provider: openrouter
      model: gpt-5-mini
    effort: &worker_effort max

Consumption is unchanged: each slot still takes a plain alias, because an alias copies a whole node and defaults.<role> requires exactly {provider, model} while effort slots require a scalar. Grouping is legal because variables is opaque (the schema types it as a plain object and the parser only checks that it is a mapping); YAML resolves aliases before the parser runs, so nothing reads the bucket's shape.

simple has no effort anchor: Factory stores no effort for subagent fields, which are model-only.

No source changes. Fixture, docs, and the registry alias test updated.