Migration playbook: legacy profiles.yaml to runtime registry YAML

Automatic migration of legacy profiles.yaml maps has been removed.

Why:

• legacy profile maps only described engine-setting overlays,
• runtime.step_settings_patch no longer exists,
• applications now own final StepSettings and provider credentials outside the profile registry.

What to do instead:

1. Create a new single-registry YAML file.
2. Rebuild each profile around runtime.system_prompt, runtime.tools, and runtime.middlewares.
3. Move model/provider/client settings into app config, env, or explicit fromConfig(...) calls.

Minimal example:

slug: default
profiles:
  analyst:
    slug: analyst
    runtime:
      system_prompt: You are an analyst.
      tools:
        - calculator