Pass s3.part_size to the multipart uploader (closes #232)

s3.part_size was loaded and defaulted but never reached the S3 client,
whose uploader used a fixed 10MiB part. The client now takes the part
size from the config, for storage_url and for the s3.* fields, and
config load rejects a value below 5MiB or above 5GiB, the part sizes S3
accepts. The docs gave the default as 5MB, which the config file reads
as 5,000,000 bytes, below the minimum; they now say 5MiB.

Judgement call: the 5GiB maximum is enforced along with the 5MiB
minimum the issue names.
Trap: at the 5MiB default the uploader's 10,000-part limit caps one
upload at about 48.8GiB, down from about 97.7GiB; blob_size_limit is
not checked against it.

Model: opus-5-5

internal/cli/config.go

@@ -205,7 +205,7 @@ storage_url: ""
 #   access_key_id: YOUR_ACCESS_KEY
 #   secret_access_key: YOUR_SECRET_KEY
 #   # region: us-east-1        # Default: us-east-1
-#   # part_size: 5MB           # Multipart upload part size. Default: 5MB
+#   # part_size: 5MiB          # Upload part size, 5MiB to 5GiB. Default: 5MiB
 #   # For the s3:// form, disable TLS with ?ssl=false in the URL, not use_ssl.
 
 # ─── OPTIONAL ────────────────────────────────────────────────────────────────