Feature: Add JSON Output Mode

## Title

Add `--json` flag to CLI commands

## Why

Many users may want to integrate DevOps-OS into automation pipelines.

## Example:

```
python -m cli.devopsos scaffold gha --json
```

Instead of printing text, return structured JSON.

Example output:

```
{
  "workflow": ".github/workflows/my-app.yml",
  "type": "github_actions"
}
```