Changes from the original:
- Explicit [DONE:n] tag tracking (more accurate than auto-marking on tool_result)
- Plan: header requirement - only extracts todos from 'Plan:' sections
- Utils extracted to separate file for testability
- Better session resume - only scans messages after plan-mode-execute marker
- Context filtering - properly filters plan-mode-context custom type messages
- Refactored to directory structure (index.ts + utils.ts + README.md)
The original auto-completed steps on every tool_result, which was inaccurate
for multi-tool steps. This version uses explicit [DONE:n] markers that the
agent outputs after completing each step.
