Add: Countdown Timer CLI

## Project proposal

I'd like to add a **Countdown Timer CLI** written in Python.

### What it does
- Takes a duration (seconds or MmSs / minutes)
- Shows a live countdown in the terminal
- Prints a clear message when time is up
- Pure Python standard library, under 100 lines

### Why
Useful mini utility for focus sessions and cooking timers — fits the repo's under-100-lines spirit.

Please assign this to me if it looks good. I'll open a PR next.

Start by inspecting the repository's existing Python mini-project layout and CLI utilities, since no specific file or test is named. Implement a standard-library countdown accepting seconds or MmSs/minutes, showing live terminal progress, and printing a clear completion message. Done means the utility stays under 100 lines and handles the listed duration forms.

Written by the indexing model from the issue text.