Default a scheme-less s3.* endpoint to TLS (closes #158)

With the s3.* config form and an endpoint written without a scheme, use_ssl being omitted built an http:// endpoint, while config.example.yml documented use_ssl as defaulting to true. Over plain HTTP a network observer sees manifests, object names, sizes and the access key id, and can alter responses.

use_ssl is now *bool: omitted (nil) means the default, TLS; only an explicit use_ssl: false forces plain HTTP. This matches the s3:// URL form, which already defaults to TLS. The config init template dropped its misleading use_ssl line from the s3:// block (that key is never read for URLs; ?ssl=false controls TLS there) and points at ?ssl=false instead.

Model: opus-4-8

internal/config/config.go

@@ -162,8 +162,10 @@ type S3Config struct {
 	AccessKeyID     string `yaml:"access_key_id"`
 	SecretAccessKey string `yaml:"secret_access_key"`
 	Region          string `yaml:"region"`
-	UseSSL          bool   `yaml:"use_ssl"`
-	PartSize        Size   `yaml:"part_size"`
+	// UseSSL selects HTTPS for a scheme-less endpoint. Omitted (nil) means
+	// the default, TLS; set it to false only to force plain HTTP.
+	UseSSL   *bool `yaml:"use_ssl"`
+	PartSize Size  `yaml:"part_size"`
 }
 
 // Path wraps the config file path for fx dependency injection.