Graceful CLI interrupt handling on Ctrl+C

[dcramer]

When the user presses Ctrl+C during a CLI scan, the process should soft-exit gracefully instead of hard terminating.

Desired behavior:

• Catch SIGINT signal
• Stop any in-progress skill scans
• Render partial results from completed/in-progress skills
• Show a clear message indicating the run was interrupted (e.g., 'Interrupted - showing partial results')
• Exit with appropriate exit code

Current behavior:

• Hard exit with no output

Implementation notes:

• Need to track which skills have completed vs in-progress
• Aggregate partial findings from whatever has finished
• Use same output rendering paths but with interrupted state flag