feat: 添加时区设置功能，默认时区改为 CST-8

- 新增 set_timezone LLM 工具，支持通过对话设置时区
- 新增 set_timezone / timezone_show CLI 命令
- 默认时区从 PST 改为 CST-8（中国标准时间 UTC+8）
- 支持 POSIX 格式和 18 个城市名映射（Asia/Shanghai 等）
- 时区通过 NVS 持久化存储（system_config namespace）
- config_show 中显示当前时区配置
- 更新 changelog.md 和 taolun.md 文档

main/mimi_secrets.h.example

@@ -21,16 +21,32 @@
 #define MIMI_SECRET_FEISHU_APP_ID   ""
 #define MIMI_SECRET_FEISHU_APP_SECRET ""
 
-/* Anthropic API */
+/* LLM Configuration
+ *
+ * Method 1: Generic API key (works for any provider)
+ *   Set MIMI_SECRET_API_KEY + MIMI_SECRET_MODEL_PROVIDER + MIMI_SECRET_MODEL
+ *
+ * Method 2: Provider-specific API keys (recommended for multi-provider setups)
+ *   Set MIMI_SECRET_<PROVIDER>_API_KEY for each provider you want to use
+ *   Switch providers via onboard portal or CLI: set_model_provider <provider>
+ *
+ * Priority: NVS (runtime config) > provider-specific key > generic key
+ */
 #define MIMI_SECRET_API_KEY         ""
-#define MIMI_SECRET_MODEL           ""
+#define MIMI_SECRET_MODEL           "claude-opus-4-5"
 #define MIMI_SECRET_MODEL_PROVIDER  "anthropic"
 
-/* SiliconFlow API (OpenAI-compatible) */
+/* Anthropic (Claude) */
+#define MIMI_SECRET_ANTHROPIC_API_KEY   ""
+
+/* OpenAI (GPT) */
+#define MIMI_SECRET_OPENAI_API_KEY      ""
+
+/* SiliconFlow (硅基流动, OpenAI-compatible) */
 #define MIMI_SECRET_SILICONFLOW_API_KEY   ""
 #define MIMI_SECRET_SILICONFLOW_BASE_URL  "https://api.siliconflow.cn/v1"
 
-/* Volcengine API (OpenAI-compatible, ByteDance Doubao models) */
+/* Volcengine (火山引擎/豆包, OpenAI-compatible) */
 #define MIMI_SECRET_VOLCENGINE_API_KEY    ""
 #define MIMI_SECRET_VOLCENGINE_BASE_URL   "https://ark.cn-beijing.volces.com/api/v3"