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/agent/context_builder.c

@@ -14,8 +14,12 @@ static size_t append_file(char *buf, size_t size, size_t offset, const char *pat
     FILE *f = fopen(path, "r");
     if (!f) return offset;
 
-    if (header && offset < size - 1) {
-        offset += snprintf(buf + offset, size - offset, "\n## %s\n\n", header);
+    if (offset >= size) return offset;
+
+    if (header) {
+        int ret = snprintf(buf + offset, size - offset, "\n## %s\n\n", header);
+        if (ret > 0) offset += (size_t)ret;
+        if (offset >= size) { offset = size - 1; buf[offset] = '\0'; fclose(f); return offset; }
     }
 
     size_t n = fread(buf + offset, 1, size - offset - 1, f);
@@ -79,23 +83,35 @@ esp_err_t context_build_system_prompt(char *buf, size_t size)
     /* Long-term memory */
     char mem_buf[4096];
     if (memory_read_long_term(mem_buf, sizeof(mem_buf)) == ESP_OK && mem_buf[0]) {
-        off += snprintf(buf + off, size - off, "\n## Long-term Memory\n\n%s\n", mem_buf);
+        if (off < size) {
+            int ret = snprintf(buf + off, size - off, "\n## Long-term Memory\n\n%s\n", mem_buf);
+            if (ret > 0) off += (size_t)ret;
+            if (off >= size) off = size - 1;
+        }
     }
 
     /* Recent daily notes (last 3 days) */
     char recent_buf[4096];
     if (memory_read_recent(recent_buf, sizeof(recent_buf), 3) == ESP_OK && recent_buf[0]) {
-        off += snprintf(buf + off, size - off, "\n## Recent Notes\n\n%s\n", recent_buf);
+        if (off < size) {
+            int ret = snprintf(buf + off, size - off, "\n## Recent Notes\n\n%s\n", recent_buf);
+            if (ret > 0) off += (size_t)ret;
+            if (off >= size) off = size - 1;
+        }
     }
 
     /* Skills */
     char skills_buf[2048];
     size_t skills_len = skill_loader_build_summary(skills_buf, sizeof(skills_buf));
     if (skills_len > 0) {
-        off += snprintf(buf + off, size - off,
-            "\n## Available Skills\n\n"
-            "Available skills (use read_file to load full instructions):\n%s\n",
-            skills_buf);
+        if (off < size) {
+            int ret = snprintf(buf + off, size - off,
+                "\n## Available Skills\n\n"
+                "Available skills (use read_file to load full instructions):\n%s\n",
+                skills_buf);
+            if (ret > 0) off += (size_t)ret;
+            if (off >= size) off = size - 1;
+        }
     }
 
     ESP_LOGI(TAG, "System prompt built: %d bytes", (int)off);