fetch: client timeout, retry with backoff, url.JoinPath (closes #63)

fetch now makes every request through an http.Client with a time
limit, ten minutes by default and set with --timeout, which must be
greater than zero. A connection error, a timeout, or a 5xx or 429
response is retried, up to five tries in all, after a random wait
whose limit doubles from one second, or after the wait the server's
Retry-After asks for, up to one minute. Each try of a file starts a
new temp file, so a retry never keeps a partial file; the size and
hash checks are unchanged. Manifest and file URLs are built with
URL.JoinPath, so a base URL with or without a trailing slash or with
a query string, and names that need escaping, all work.

Model: opus-5-5

internal/cli/fetch.go

@@ -281,9 +281,7 @@ func (mfa *CLIApp) fetchManifestOperation(ctx *cli.Context) error {
 		return errInvalidTimeout
 	}
 
-	inputURL := ctx.Args().Get(0)
-
-	manifestURL, err := resolveManifestURL(inputURL)
+	manifestURL, err := resolveManifestURL(ctx.Args().Get(0))
 	if err != nil {
 		return fmt.Errorf("invalid URL: %w", err)
 	}