Implement Issues

Build open issues with agents, one pull request per issue. Each issue goes through the flow a person would take it through:

1. a spec and plan, written by Superpowers from the issue;
2. the build, done by Superpowers;
3. a pull request and a code review;

except that no one answers questions along the way. Every decision an agent makes on the user's behalf lands in its pull request, where the user reviews it.

Two agents work on each issue. A planner writes the spec and plan. A build lead executes the plan and answers the review. When plan approval is on, the user reviews the spec and plan on a draft pull request before the build lead starts, and a later run picks up what they approved.