refactor: split CLI module into focused formatters and groupers

- Created cli/groupers/ViolationGrouper.ts for severity filtering
- Created cli/formatters/OutputFormatter.ts for violation formatting
- Created cli/formatters/StatisticsFormatter.ts for metrics display
- Reduced cli/index.ts from 484 to 260 lines (46% reduction)
- All 345 tests pass, CLI output identical to before
- No breaking changes

packages/guardian/src/cli/groupers/ViolationGrouper.ts

@@ -0,0 +1,29 @@
+import { SEVERITY_ORDER, type SeverityLevel } from "../../shared/constants"
+
+export class ViolationGrouper {
+    groupBySeverity<T extends { severity: SeverityLevel }>(
+        violations: T[],
+    ): Map<SeverityLevel, T[]> {
+        const grouped = new Map<SeverityLevel, T[]>()
+
+        for (const violation of violations) {
+            const existing = grouped.get(violation.severity) ?? []
+            existing.push(violation)
+            grouped.set(violation.severity, existing)
+        }
+
+        return grouped
+    }
+
+    filterBySeverity<T extends { severity: SeverityLevel }>(
+        violations: T[],
+        minSeverity?: SeverityLevel,
+    ): T[] {
+        if (!minSeverity) {
+            return violations
+        }
+
+        const minSeverityOrder = SEVERITY_ORDER[minSeverity]
+        return violations.filter((v) => SEVERITY_ORDER[v.severity] <= minSeverityOrder)
+    }
+}