Add a configurable mention filter to the Signal adapter so the bot
only responds in groups when it is explicitly @mentioned.
Changes:
- gateway/platforms/signal.py: read require_mention from adapter
extra config or SIGNAL_REQUIRE_MENTION env var; skip group messages
that don't mention the bot account (checked in rendered text and
raw mention metadata)
- gateway/config.py: map signal.require_mention YAML key to the
SIGNAL_REQUIRE_MENTION env var (env var takes precedence)
Config example:
signal:
require_mention: true
Or via env var:
SIGNAL_REQUIRE_MENTION=true
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