Registry scans can overwrite their input or comparison report #756

On main at b6f0304, the registry scan branch returns before the same-file output guard added in #729. As a result, a successful scan can replace its own input with the generated report.

Reproduction (no LLM required):

printf '{"servers":[]}\n' > registry.json
skillspector scan registry.json --mcp-registry --format json --output registry.json

Expected: reject the output path before scanning and preserve the input bytes.
Actual: exit 0 and replace registry.json with the scan report.

The same issue affects an existing comparison report passed with --mcp-registry-compare previous-report.json --output previous-report.json. Symlink and hardlink aliases can also refer to the same file.

I can reuse the existing destination check for both registry inputs before invoking the scanner, with regression tests for path aliases and separate-output success.