spec done — mark a task complete

CLI at scripts/specctl. Three flavors:

• Mark a specific task done — user names a task and says it's finished.
• Discover potentially done tasks — user wants to know which in-progress tasks look complete based on git evidence.
• Verify then mark — user wants tests/lint/build to pass before changing status.

Mark a specific task

Step 1: Find the task

Look up the task file under .spec/tasks/ matching the user-supplied id (add the TASK- prefix if missing). If not found, say "Task not found." and stop.

Step 2: Check current status

If the task is already done, say "Already complete." and stop. Otherwise continue.

Step 3: Verify completion evidence