Add --quiet for report-only CI use

[amic25]

Goal

Add a --quiet option that suppresses the “Report written” status line while preserving generated reports, errors, and exit codes.

Scope

• Add the option to agentguard scan.
• Keep errors visible.
• Do not change JSON, Markdown, or SARIF file contents.
• Add CLI tests and a README example.

Acceptance criteria

• --quiet suppresses only nonessential status output.
• Findings and failure thresholds behave exactly as before.
• make check passes.

See CONTRIBUTING.md before starting.