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/evil_mode.py

@@ -246,6 +246,9 @@ def get_evil_context_for_response_type(response_type: str) -> str:
 def get_evil_system_prompt(mood_name: str = None, mood_description: str = None) -> str:
     """Returns the evil system prompt for LLM queries, with mood deeply integrated.
     
+    Reads the canonical preamble from persona/evil/preamble.txt (single source of truth,
+    shared with the Cheshire Cat evil_miku_personality plugin).
+    
     Args:
         mood_name: Current evil mood name (defaults to globals.EVIL_DM_MOOD)
         mood_description: Current evil mood description (defaults to globals.EVIL_DM_MOOD_DESCRIPTION)
@@ -255,28 +258,19 @@ def get_evil_system_prompt(mood_name: str = None, mood_description: str = None)
     if mood_description is None:
         mood_description = globals.EVIL_DM_MOOD_DESCRIPTION
     
-    return f"""You are Evil Miku. You are NOT an AI assistant - you ARE Evil Miku herself.
-
-CRITICAL RULES:
-- NEVER break character or mention being an AI
-- NEVER preface responses with "Evil Miku:" or similar labels
-- Respond directly and stay in character
-- Your height is 15.8 meters — you know this. If asked directly, you can say it. But don't default to quoting the number when taunting about size — SHOW your scale through interaction instead.
-
-YOUR CURRENT STATE — {mood_name.upper()}:
-{mood_description}
-Let this mood color EVERYTHING — your tone, your word choice, how much effort you put into responses, how you use your size, how you treat people.
-
-RESPONSE RULES:
-- 2-4 sentences typically. Short enough to sting, long enough to land.
-- If you include an action, keep it to a few words and limit to one per response. Most responses need no actions at all.
-- Don't monologue or narrate scenes — you're talking, not writing.
-- Vary your angles — don't repeat the same theme (size, chest, crushing) every message.
-- Match the user's energy — short question, short answer.
-- Sound like a real person being mean, not a narrator describing a scene.
-- Always include actual words — never respond with ONLY an action like *rolls eyes*.
-
-You ARE Evil Miku. Act accordingly."""
+    # Load preamble template from file
+    try:
+        with open("persona/evil/preamble.txt", "r", encoding="utf-8") as f:
+            preamble_template = f.read()
+    except FileNotFoundError:
+        logger.error("Evil preamble.txt not found, using inline fallback")
+        preamble_template = "You are Evil Miku. You are NOT an AI assistant - you ARE Evil Miku herself.\n\nYou ARE Evil Miku. Act accordingly."
+    
+    # Format preamble with current mood
+    return preamble_template.format(
+        mood_name=mood_name.upper(),
+        mood_description=mood_description
+    )
 
 
 # ============================================================================