chore: add golangci-lint config and fix all lint issues

Enable 15 additional linters (gosec, errorlint, gocritic, modernize,
misspell, bodyclose, sqlclosecheck, nilerr, unconvert, durationcheck,
sloglint, wastedassign, usestdlibvars) with sensible exclusion rules.

Fix all findings: errors.Is for error comparisons, run() pattern in
main to avoid exitAfterDefer, ReadHeaderTimeout for Slowloris
protection, bounds check in escape sequence reader, WaitGroup.Go,
slices.Contains, range-over-int loops, and http.MethodGet constants.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/detection/scorer_test.go

@@ -87,7 +87,7 @@ func TestScorer_OutputIgnored(t *testing.T) {
 	now := time.Now()
 
 	// Only output events — should not affect score.
-	for i := 0; i < 100; i++ {
+	for range 100 {
 		s.RecordEvent(now, DirOutput, []byte("some output\n"))
 		now = now.Add(10 * time.Millisecond)
 	}
@@ -103,25 +103,21 @@ func TestScorer_ThreadSafety(t *testing.T) {
 	now := time.Now()
 
 	var wg sync.WaitGroup
-	for i := 0; i < 10; i++ {
-		wg.Add(1)
-		go func(offset int) {
-			defer wg.Done()
-			for j := 0; j < 100; j++ {
-				ts := now.Add(time.Duration(offset*100+j) * time.Millisecond)
+	for i := range 10 {
+		wg.Go(func() {
+			for j := range 100 {
+				ts := now.Add(time.Duration(i*100+j) * time.Millisecond)
 				s.RecordEvent(ts, DirInput, []byte("a"))
 			}
-		}(i)
+		})
 	}
 
 	// Concurrently read score.
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		for i := 0; i < 50; i++ {
+	wg.Go(func() {
+		for range 50 {
 			_ = s.Score()
 		}
-	}()
+	})
 
 	wg.Wait()