Add memory usage summary in scan output

## Context
zclean reports how many processes it found/killed. Adding a memory summary (total MB freed) would make the value more tangible.

## What to do
After scanning, sum up RSS memory of zombie processes and display:
`Found 5 zombie processes using 2.1 GB RAM`

## Files to modify
- `src/scanner.js` (collect memory info)
- `src/reporter.js` (display summary)

## Difficulty: Easy