Support multiple output formats from a single scan

[drogers0]

Problem

Currently --format accepts only a single value (json, html, or sarif). To get reports in multiple formats, users must run the scan multiple times, duplicating potentially expensive embedding and similarity work.

Expected behavior

A single scan should be able to produce multiple output formats, e.g.:

clonehunter scan . --format json html
clonehunter scan . --format all

Each format would write to its default filename (clonehunter_report.json, clonehunter_report.html, etc.) or to paths specified via --out.

Scope

This applies to both scan and diff subcommands.