New rss bot type: on start it creates a broadcast channel (observer group with
recent history on) and polls a configured feed URL; new posts are broadcast to
the channel. Subscribers join the channel link (seen on the bot's profile);
direct contacts get a welcome + the latest items and can send /new for the
latest. Stdlib-only feed parsing (urllib + ElementTree), seeds existing items on
startup so it doesn't replay the whole feed. Config: feed_url, poll_seconds.
Adds rss_test.py (mock feed) — passes.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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