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>

bot.py

@@ -112,6 +112,7 @@ class BCSBot(commands.Bot):
             window_size=sentiment.get("rolling_window_size", 10),
             window_minutes=sentiment.get("rolling_window_minutes", 15),
             offense_reset_minutes=timeouts.get("offense_reset_minutes", 120),
+            warning_expiration_minutes=timeouts.get("warning_expiration_minutes", 30),
         )
 
     def get_mode_config(self) -> dict: