Implement Plan

This is for executing implementation plans you've already written and saved in your scratchpad. It reads the plan file, checks what's already done, and works through each phase systematically. The interesting bit is how it handles reality not matching the plan: it's designed to stop and ask rather than improvise, which makes sense when you're following a detailed technical spec. It updates checkboxes as it goes so you can resume work cleanly. Useful when you've done the thinking work upfront and want to hand off the actual implementation to Claude without it going off script.