implement acmq: read ACM Queue articles via RSS

Three commands: top (recent articles), topics (static taxonomy), and
article <id-or-url> (single article by id). Data comes from the public
RSS feed at queue.acm.org/rss/feeds/queuecontent.xml — no auth, no
key, works from any IP. Config-based client with BaseURL test seam,
paced requests, and exponential-backoff retries.