feat: add warning expiration and exclude moderated messages from context

Warning flag now auto-expires after a configurable duration
(warning_expiration_minutes, default 30m). After expiry, the user must
be re-warned before a mute can be issued.

Messages that triggered moderation actions (warnings/mutes) are now
excluded from the LLM context window in both buffered analysis and
mention scans, preventing already-actioned content from influencing
future scoring. Uses in-memory tracking plus bot reaction fallback
for post-restart coverage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config.yaml

@@ -44,6 +44,7 @@ timeouts:
   escalation_minutes: [30, 60, 120, 240]  # Escalating timeout durations
   offense_reset_minutes: 1440  # Reset offense counter after this much good behavior (24h)
   warning_cooldown_minutes: 5  # Don't warn same user more than once per this window
+  warning_expiration_minutes: 30  # Warning expires after this long — user must be re-warned before mute
 
 messages:
   warning: "Easy there, {username}. The Breehavior Monitor is watching. \U0001F440"