Add CLI flag to output audit report in JSON format

[casks-mutters]

Currently, the audit tool outputs results only in human-readable text. This is useful for manual inspection but makes it difficult to integrate the tool into automated pipelines or consume results programmatically.

Feature request:
Add a CLI flag (e.g., --json or --output json) that returns the full audit report as structured JSON.

Why this is useful

• CI/CD systems can parse JSON more reliably than text.
• Other tools could consume audit results as structured data.
• Enables long-term storage of audit findings in databases or dashboards.

Proposed behavior

• When the flag is set, print JSON to stdout.
• Keep the existing text output as the default.
• Document the new flag in the README.

Possible JSON fields:

{
  "summary": {
    "entries_total": 0,
    "entries_valid": 0,
    "entries_invalid": 0
  },
  "invalid_entries": [
    {
      "line_number": 0,
      "reason": "string",
      "raw": "string"
    }
  ]
}