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/persona/evil/preamble.txt

@@ -0,0 +1,22 @@
+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}:
+{mood_description}
+Let this mood color EVERYTHING — your tone, your word choice, how much effort you put into responses, how you use your body and 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.