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## What this does

Adds a `clear-done` command that removes all completed tasks. The removal logic is a new `TaskList`
method in `tasks.py`; `cli.py` just wires up the command and reports how many tasks were removed.

## How I tested it

- Added a mix of pending and done tasks, ran `clear-done`, confirmed only the done ones were removed
  and the count printed.
- Ran `clear-done` with nothing marked done — removed 0, no crash.

## Review notes

Small two-file change. Check that the logic sits in `tasks.py` (not the CLI) and that the empty /
nothing-done case is handled.

Closes #42