Renders a markdown checklist from plan steps and summaries.

Used to show the LLM its progress through a plan as user feedback messages. This is the default progress renderer used when no custom progress_fn is set on the agent definition. Custom progress_fn implementations can call render/2 directly if they want to include the checklist as part of their output.

Plan steps render as pending by default. Steps are checked off when (step-done "id" "summary") is called with a matching ID. This is optional — plans work as display-only labels without any step-done calls.

Step-done entries whose IDs don't appear in the plan are collected under an "Out-of-Plan Steps" section.