- Add Artifact type to Step (name, path, artifact_type, description)
- step_done tool accepts optional artifacts parameter
- Save artifacts to step_artifacts DB table
- Display artifacts in frontend PlanSection (tag style)
- Show artifacts in step context for sub-agents and coordinator
- Add LLM client retry with exponential backoff
Main loop becomes a coordinator that reviews step summaries and may
revise the plan. Each step gets its own chat history and scratchpad,
preventing context pollution across steps.
- Add run_step_loop with 50-iteration limit and isolated context
- Replace advance_step with step_done (sub-loop only)
- Add coordinator review after each step completion
- Add scratchpad 8K capacity check
- Add 33 unit tests for state, tools, and message building
