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# Add a `delete <index>` command to the CLI

**Type:** feature · **Priority:** p2 · **Labels:** `cli`, `ready`, `agent`

## Context

`tasks-app` can `add`, `list`, and mark a task `done`, but there's no way to remove a task. Once a
task is added by mistake it stays forever. The `done` command already takes an index and mutates the
list through a method on `TaskList`, so a `delete` command should follow the exact same shape. This
is a patterned change, not a design problem.

## Acceptance criteria

- `python3 cli.py delete <index>` removes the task at that 0-based index and saves the list.
- After deleting, the remaining tasks keep their relative order.
- `delete` with an out-of-range or non-integer index prints a clear error (e.g.
  `no task at index 99`) and exits non-zero, instead of dumping a traceback.
- The logic lives on `TaskList` (a `remove(index)` method or equivalent), mirroring how `complete`
  works; `cli.py` only parses arguments and calls it.
- A test covers: a successful delete removes the right task, and an out-of-range delete is handled.

## Out of scope

- Changing how tasks are stored or numbered.
- Bulk delete, undo, or a confirmation prompt.
- Reworking the existing `add` / `list` / `done` commands.
