add --json flag for structured CLI output

[Giuseppe552]

The CLI currently outputs JSON by default, but plugin progress logs go to stderr mixed with the output.

Goal: Add a `--json` flag that outputs structured NDJSON (one JSON object per event):
```
{"type":"plugin","id":"github","status":"done","nodes":3,"edges":2}
{"type":"plugin","id":"dns","status":"done","nodes":8,"edges":7}
{"type":"resolve","merged":1,"suggested":0}
{"type":"done","nodes":47,"edges":63}
```

This makes the CLI pipeable into monitoring tools and dashboards.