Fix cross-bot infinite reply loop: honor the existing other_bots config

Fix cross-bot infinite reply loop: honor the existing other_bots config

muc_message() only ever ignored the bot's own nick. With multiple sibling
bot instances (urlbug, pibug, urlbrot) in the same room, each correctly
ignores itself but happily reacts to the others' replies - discovered
live when a smoke-test message containing a CVE id caused
SecurityTracker's URL reply to be picked up by the other bot's
URLResolver, whose title-scrape reply contained the same CVE id, causing
SecurityTracker to fire again, forever, across both bots, hammering
security-tracker.debian.org indefinitely until manually restarted.

persistent_config.ini.spec already had an `other_bots` key seemingly
designed for exactly this, but nothing in the codebase ever read it. Wired
it into muc_message()'s existing self-nick check. Populating the actual
other_bots list per host is a separate, per-deployment config change (not
committed here, since persistent_config.ini is gitignored per-host state).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>