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/config/config.go

@@ -236,11 +236,11 @@ func Load(path string) (*Config, error) {
 // Returns an error describing the first validation failure encountered.
 func (c *Config) Validate() error {
 	if len(c.AgeRecipients) == 0 {
-		return fmt.Errorf("at least one age_recipient is required")
+		return fmt.Errorf("at least one age_recipient is required (generate with: age-keygen)")
 	}
 
 	if len(c.Snapshots) == 0 {
-		return fmt.Errorf("at least one snapshot must be configured")
+		return fmt.Errorf("at least one snapshot must be configured (see config.example.yml)")
 	}
 
 	for name, snap := range c.Snapshots {
@@ -299,7 +299,7 @@ func (c *Config) validateStorage() error {
 
 	// Legacy S3 configuration
 	if c.S3.Endpoint == "" {
-		return fmt.Errorf("s3.endpoint is required (or set storage_url)")
+		return fmt.Errorf("storage not configured; set storage_url or provide s3.endpoint + s3.bucket + credentials")
 	}
 
 	if c.S3.Bucket == "" {