feat: add stats command for system overview

## Context
Agents need a quick way to understand the current system state before deciding what to do next.

## Proposed Change
New command: `blogwatcher stats [--format json]`

Output includes:
- `total_blogs`: number of tracked blogs
- `total_articles`: total article count
- `unread_articles`: count of unread articles
- `last_scan`: timestamp of most recent scan across all blogs