fix(pattern): add nil check and fmt import for GetPatternInsight

- [x] Add `fmt` import for error formatting
- [x] Add nil check for pattern before using it
- [x] Remove duplicate comment line

internal/pattern/detector.go

@@ -3,6 +3,7 @@ package pattern
 
 import (
 	"context"
+	"fmt"
 	"sort"
 	"strings"
 	"sync"
@@ -370,12 +371,16 @@ func (d *Detector) CandidateCount() int {
 	return len(d.candidates)
 }
 
+// GetPatternInsight returns a formatted insight string for a pattern.
 // GetPatternInsight returns a formatted insight string for a pattern.
 func (d *Detector) GetPatternInsight(ctx context.Context, patternID int64) (string, error) {
 	pattern, err := d.patternStore.GetPatternByID(ctx, patternID)
 	if err != nil {
 		return "", err
 	}
+	if pattern == nil {
+		return "", fmt.Errorf("pattern not found: %d", patternID)
+	}
 
 	return formatPatternInsight(pattern), nil
 }