Route scanner output through writer, fix S3 error handling, improve error messages

Scanner now writes all user-facing output to an io.Writer (os.Stdout
when progress is enabled, io.Discard in --cron mode). This fixes the
long-standing issue where --cron still printed progress lines.

S3 HeadObject now properly distinguishes not-found from other errors
instead of swallowing all errors as not-found.

Config/CLI error messages include actionable hints (where to find the
config, how to generate keys, what storage options exist).

internal/s3/client.go

@@ -2,6 +2,7 @@ package s3
 
 import (
 	"context"
+	"errors"
 	"io"
 	"sync/atomic"
 
@@ -10,6 +11,7 @@ import (
 	"github.com/aws/aws-sdk-go-v2/credentials"
 	"github.com/aws/aws-sdk-go-v2/feature/s3/manager"
 	"github.com/aws/aws-sdk-go-v2/service/s3"
+	s3types "github.com/aws/aws-sdk-go-v2/service/s3/types"
 	"github.com/aws/smithy-go/logging"
 )
 
@@ -203,9 +205,12 @@ func (c *Client) HeadObject(ctx context.Context, key string) (bool, error) {
 		Key:    aws.String(fullKey),
 	})
 	if err != nil {
-		// Check if it's a not found error
-		// TODO: Add proper error type checking
-		return false, nil
+		var notFound *s3types.NotFound
+		var noSuchKey *s3types.NoSuchKey
+		if errors.As(err, &notFound) || errors.As(err, &noSuchKey) {
+			return false, nil
+		}
+		return false, err
 	}
 	return true, nil
 }