Allow a step in the TOML to define multiple sequential sub-prompts that execute within a single persistent SDK session, sharing Anthropic's server-side context between them rather than rebuilding context manually each time.
A step with sub-prompts should look like a normal step in the TOML but with a sub_prompts list instead of a single prompt field. All sub-prompts run in order within one session. Only the final sub-prompt's output — or an optional session_summary_prompt that the user can define — gets appended to the rolling pipeline context for downstream steps.
This only applies to backend = "anthropic" steps. Ollama steps remain single-prompt only as they have no session continuity. If a step has both prompt and sub_prompts defined, raise a validation error at preflight.
Update the TOML schema, config dataclasses, loader validation, runner session handling, and the example config to reflect this. Do not change anything else.