Speckit Implement

This is the execution engine for the Spec Kit workflow. It picks up where speckit-tasks leaves off, reading your tasks.md and plan.md to actually implement the feature. The workflow is methodical: verify prerequisites, check any checklists for completion (and pause if items are incomplete), load all your specs and contracts, then execute tasks phase by phase respecting dependencies and parallel execution markers. It follows TDD when contracts exist, handles ignore file setup for whatever stack you're using, and marks tasks complete as it goes. If you've done the planning work and have a solid task breakdown, this runs the implementation. The checklist gate is smart, it won't let you proceed with incomplete validation items unless you explicitly override it.