Load system prompt from persistent file

- Load system prompt from data/system-prompt.txt (persistent)
- Fallback to AI_SYSTEM_PROMPT env variable
- Fallback to hardcoded default
- File can be edited without rebuilding container
- Includes default UlfBot/SchrottBot personality

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

CLAUDE.md

@@ -61,7 +61,9 @@ src/
 
 ### AI Client (ai.ts)
 - OpenAI-compatible API (works with OpenAI, Ollama, OpenRouter, etc.)
-- Prompt system: Global (.env) + Group-specific (DB)
+- Prompt system:
+  - Global: `data/system-prompt.txt` → `.env` fallback → default
+  - Group-specific: stored in DB per chat
 - API key masking for security
 - Context: Last 50 messages
 
@@ -78,6 +80,14 @@ BOT_TOKEN=                    # Telegram bot token (required)
 AI_DEFAULT_API_KEY=           # Default API key (optional)
 AI_DEFAULT_BASE_URL=          # Default API URL (default: OpenAI)
 AI_DEFAULT_MODEL=             # Default model (default: gpt-4o-mini)
+AI_SYSTEM_PROMPT=             # Global system prompt (fallback, see below)
+```
+
+**System Prompt Loading:**
+1. `data/system-prompt.txt` (persistent, editable without rebuild)
+2. `AI_SYSTEM_PROMPT` env variable (fallback)
+3. Hardcoded default (last resort)
+AI_DEFAULT_MODEL=             # Default model (default: gpt-4o-mini)
 AI_SYSTEM_PROMPT=             # Global system prompt
 ```