Fix wrong-prefix silent ignore and Minimax image understand host

- Remove "Use .cmd for this role" reply when owner uses wrong prefix;
  wrong-prefix commands are now silently ignored for all users
- Fix AIImageUnderstandConfig default api_host to api.minimax.chat
  (minimax-coding-plan-mcp routes all calls there, not api.minimax.io)

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

.env.example

@@ -2,4 +2,4 @@ TG_API_ID=123456
 TG_API_HASH=your_api_hash_here
 SESSION_DIR=sessions
 ACCOUNTS_JSON=[{"name":"personal","session":"personal"},{"name":"work","session":"work"}]
-# AI_CONFIG={"default_model":"mm","context_messages":12,"prompt":{"root":"prompts/ai","system_file":"system.md","parts_dir":"parts"},"image_understand":{"api_key":"your_minimax_token_plan_key_here","api_host":"https://api.minimax.io","command":["uvx","minimax-coding-plan-mcp","-y"]},"providers":{"minimax":{"api_key":"your_minimax_api_key_here","base_url":"https://api.minimax.chat/v1"},"openrouter":{"api_key":"your_openrouter_api_key_here","base_url":"https://openrouter.ai/api/v1"}},"models":{"mm":{"provider":"minimax","model":"MiniMax-Text-01"},"or":{"provider":"openrouter","model":"openai/gpt-4o-mini"}}}
+# AI_CONFIG={"default_model":"mm","context_messages":12,"prompt":{"root":"prompts/ai","system_file":"system.md","parts_dir":"parts"},"image_understand":{"api_key":"your_minimax_token_plan_key_here","api_host":"https://api.minimax.chat","command":["uvx","minimax-coding-plan-mcp","-y"]},"providers":{"minimax":{"api_key":"your_minimax_api_key_here","base_url":"https://api.minimax.chat/v1"},"openrouter":{"api_key":"your_openrouter_api_key_here","base_url":"https://openrouter.ai/api/v1"}},"models":{"mm":{"provider":"minimax","model":"MiniMax-Text-01"},"or":{"provider":"openrouter","model":"openai/gpt-4o-mini"}}}