/speckit.implement — execute the task list

Gate: tasks.md exists for the current feature (run python3 speckit.py status to confirm); if it is missing, stop and run /speckit.tasks first.

Read spec.md, plan.md and tasks.md of the current feature.
Work through tasks.md strictly top to bottom, phase by phase: implement each task test-first, and flip its checkbox to - [x] only after its verification actually passed.
Never renumber or delete tasks; append newly discovered work as new checkboxes at the end of the fitting phase.
When all boxes are checked, run python3 speckit.py check and report the result to the user together with how to run the built feature.