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/cli/config.go

@@ -192,8 +192,8 @@ storage_url: ""
 #   access_key_id: YOUR_ACCESS_KEY
 #   secret_access_key: YOUR_SECRET_KEY
 #   # region: us-east-1        # Default: us-east-1
-#   # use_ssl: true            # Default: true
 #   # part_size: 5MB           # Multipart upload part size. Default: 5MB
+#   # For the s3:// form, disable TLS with ?ssl=false in the URL, not use_ssl.
 
 # ─── OPTIONAL ────────────────────────────────────────────────────────────────