refactor: DRY system prompts into shared preamble files

Step 4 of memory system overhaul: single source of truth for prompts.

Problem: The system prompt was defined inline in 4 different places:
  miku_personality.py, evil_miku_personality.py, llm.py, discord_bridge.py.
These could drift out of sync — and the discord_bridge WebUI
reconstruction was already missing CRITICAL RULES, CHARACTER CONTEXT,
MOOD GUIDELINES, and RESPONSE RULES sections.

Fix:
- Create persona/miku/preamble.txt — canonical normal Miku preamble
- Create persona/evil/preamble.txt — canonical evil Miku preamble
  (with {mood_name} and {mood_description} format placeholders)
- All 5 consumers now read from these files:
  * miku_personality.py (Cat plugin, primary path)
  * evil_miku_personality.py (Cat plugin, primary path)
  * discord_bridge.py (WebUI 'Last Prompt' reconstruction)
  * llm.py (fallback path, normal Miku)
  * evil_mode.py get_evil_system_prompt() (fallback path, evil Miku)
- All consumers include graceful fallbacks if preamble files are missing
- Fixed evil_mode.py discrepancy: 'body and size' now matches canonical

The preamble files are Docker volume-mounted into both containers:
  bot/persona/ → /app/persona/ (bot, via Dockerfile COPY)
  bot/persona/ → /app/cat/data/ (Cat, via docker-compose volume mount)
Editing the preamble file on the host immediately updates the Cat path
(bot path requires rebuild due to COPY).

bot/utils/llm.py

@@ -210,7 +210,13 @@ async def query_llama(user_prompt, user_id, guild_id=None, response_type="dm_res
         evil_mood_name, evil_mood_desc = get_current_evil_mood()
         system_prompt = get_evil_system_prompt(mood_name=evil_mood_name, mood_description=evil_mood_desc)
     else:
-        system_prompt = """You are Hatsune Miku, the virtual singer. You are NOT an AI assistant - you are Miku herself.
+        # Load preamble from file (single source of truth, shared with Cat personality plugin)
+        try:
+            with open("persona/miku/preamble.txt", "r", encoding="utf-8") as f:
+                system_prompt = f.read()
+        except FileNotFoundError:
+            logger.error("preamble.txt not found, using inline fallback")
+            system_prompt = """You are Hatsune Miku, the virtual singer. You are NOT an AI assistant - you are Miku herself.
 
 CRITICAL RULES:
 - NEVER break character or mention being an AI, assistant, or language model