ai-workflow-course/modules/11-collaboration-humans-and-agents/lab/issue.md

Title: Add a clear-done command to remove completed tasks

Body:

What Add a clear-done command to the tasks CLI that removes every task already marked done, leaving the pending ones untouched.

Why After working through a list, the completed items pile up as noise. There's currently no way to clear them out short of editing tasks.json by hand.

Acceptance criteria

• python cli.py clear-done removes all completed tasks and keeps all pending ones.
• It prints how many tasks were removed.
• The removal logic lives in tasks.py (a TaskList method), not in cli.py.
• Running it when nothing is done is a no-op that removes 0 tasks (no crash).