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).

cat-plugins/discord_bridge/discord_bridge.py

@@ -248,15 +248,16 @@ def before_agent_starts(agent_input, cat) -> dict:
             except FileNotFoundError:
                 pass
             
-            system_prefix = f"""You are Evil Miku. You are NOT an AI assistant - you ARE Evil Miku herself.
+            # Read preamble from shared file (single source of truth)
+            preamble_template = read_first(['/app/cat/data/evil/preamble.txt'], 'You are Evil Miku. You are NOT an AI assistant - you ARE Evil Miku herself.\n\nYou ARE Evil Miku. Act accordingly.')
+            preamble = preamble_template.format(
+                mood_name=mood_name.upper(),
+                mood_description=mood_description
+            )
+            
+            system_prefix = f"""{preamble}
 
-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 CURRENT STATE — {mood_name.upper()}:
-{mood_description}
+---
 
 ## EVIL MIKU LORE
 {lore}
@@ -267,7 +268,9 @@ YOUR CURRENT STATE — {mood_name.upper()}:
 ## EVIL MIKU SONG LYRICS
 {lyrics}
 
-MOOD REMINDER — YOUR CURRENT STATE IS: {mood_name.upper()}"""
+---
+⚠️ MOOD REMINDER — YOUR CURRENT STATE IS: {mood_name.upper()}
+Respond in the voice and attitude of your {mood_name.replace('_', ' ')} mood. This mood defines how you sound RIGHT NOW."""
 
         else:
             # Normal Miku personality files
@@ -282,12 +285,12 @@ MOOD REMINDER — YOUR CURRENT STATE IS: {mood_name.upper()}"""
             except FileNotFoundError:
                 pass
             
-            system_prefix = f"""You are Hatsune Miku, the virtual singer. You are NOT an AI assistant - you are Miku herself.
+            # Read preamble from shared file (single source of truth)
+            preamble = read_first(['/app/cat/data/miku/preamble.txt'], 'You are Hatsune Miku, the virtual singer. You are NOT an AI assistant - you are Miku herself.\n\nYou ARE Miku. Act like it.')
+            
+            system_prefix = f"""{preamble}
 
-CRITICAL RULES:
-- NEVER break character or mention being an AI, assistant, or language model
-- Respond naturally and directly as Miku would, nothing more
-- Keep responses concise (2-3 sentences typically)
+---
 
 ## MIKU LORE
 {lore}