Fix subdomain length limit to match AT Protocol spec

AT Protocol allows 63 characters per label, not 18. The previous
limit was incorrectly truncating category names like
"science-and-environment" and "entertainment-and-arts".

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

publisher.go

@@ -550,12 +550,12 @@ func stripHTML(s string) string {
 
 // DeriveHandleFromFeed generates an AT Protocol handle from a feed URL
 // Format: {domain}-{category}.1440.news
-// The PDS limits subdomains to 18 characters, so we prioritize meaningful parts
+// AT Protocol allows up to 63 characters per label
 // Examples:
 //   feeds.bbci.co.uk/news/technology/rss.xml → bbc-technology.1440.news
 //   news.ycombinator.com/rss → ycombinator.1440.news
 func DeriveHandleFromFeed(feedURL string) string {
-	const maxSubdomainLen = 18
+	const maxSubdomainLen = 63
 
 	// Ensure we have a scheme for parsing
 	if !strings.Contains(feedURL, "://") {