refactor: extract sentiment cog into package with shared _process_finding

Convert cogs/sentiment.py (1050 lines) into cogs/sentiment/ package:
- __init__.py (656 lines): core SentimentCog with new _process_finding()
  that deduplicates the per-user finding loop from _process_buffered and
  _run_mention_scan (~90 lines each → single shared method)
- actions.py: mute_user, warn_user
- topic_drift.py: handle_topic_drift
- channel_redirect.py: handle_channel_redirect, build_channel_context
- coherence.py: handle_coherence_alert
- log_utils.py: log_analysis, log_action, score_color
- state.py: save_user_state, flush_dirty_states

All extracted modules use plain async functions (not methods) receiving
bot/config as parameters. Named log_utils.py to avoid shadowing stdlib
logging. Also update CLAUDE.md with comprehensive project documentation.

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

cogs/sentiment/coherence.py

@@ -0,0 +1,43 @@
+import asyncio
+import logging
+
+import discord
+
+from cogs.sentiment.log_utils import log_action
+from cogs.sentiment.state import save_user_state
+
+logger = logging.getLogger("bcs.sentiment")
+
+
+async def handle_coherence_alert(
+    bot, message: discord.Message, degradation: dict, coherence_config: dict,
+    db_message_id: int | None, dirty_users: set[int],
+):
+    flag = degradation["flag"]
+    messages_map = coherence_config.get("messages", {})
+    alert_text = messages_map.get(flag, messages_map.get(
+        "default", "You okay there, {username}? That message was... something."
+    )).format(username=message.author.display_name)
+
+    await message.channel.send(alert_text)
+    await log_action(
+        message.guild,
+        f"**COHERENCE ALERT** | {message.author.mention} | "
+        f"Score: {degradation['current']:.2f} | Baseline: {degradation['baseline']:.2f} | "
+        f"Drop: {degradation['drop']:.2f} | Flag: {flag}",
+    )
+    logger.info(
+        "Coherence alert for %s: score=%.2f baseline=%.2f drop=%.2f flag=%s",
+        message.author, degradation["current"], degradation["baseline"],
+        degradation["drop"], flag,
+    )
+
+    asyncio.create_task(bot.db.save_action(
+        guild_id=message.guild.id,
+        user_id=message.author.id,
+        username=message.author.display_name,
+        action_type="coherence_alert",
+        message_id=db_message_id,
+        details=f"score={degradation['current']:.2f} baseline={degradation['baseline']:.2f} drop={degradation['drop']:.2f} flag={flag}",
+    ))
+    save_user_state(bot, dirty_users, message.author.id)