Feature: Allow excluding lint issues from reports #40

Users often hit known, intentional, or external findings that clutter HTML/JSON reports and make triage harder. Today, all lint results appear in the generated reports. We need a way to exclude specific issues from report output (while optionally still controlling whether excluded items affect exit status — see below).

Goals

• Users can configure exclusions.
• Excluded issues must not appear in:
  • per-plugin reports (lints.html / lints.json), and
  • aggregated/master reports (if applicable).
• Configuration should be repeatable (e.g. config file + optional CLI), documented, and testable.