优化系统提示词：移除冗余的工具列表，自动创建skills目录

- 移除 ToolsSection：工具定义已通过 API 的 tools 参数传递，无需在提示词中重复
- SkillsLoader 启动时自动创建 ~/.picobot/skills 目录

src/agent/system_prompt.rs

@@ -39,7 +39,6 @@ impl SystemPromptBuilder {
             sections: vec![
                 Box::new(ToolHonestySection),
                 Box::new(NoToolNarrationSection),
-                Box::new(ToolsSection),
                 Box::new(YourTaskSection),
                 Box::new(SafetySection),
                 Box::new(WorkspaceSection),

src/skills/mod.rs

@@ -70,6 +70,15 @@ impl SkillsLoader {
         let mut state = self.state.lock().unwrap();
         state.loaded_skills.clear();
 
+        // Ensure ~/.picobot/skills directory exists
+        if !self.picobot_skills_dir.exists() {
+            if let Err(e) = std::fs::create_dir_all(&self.picobot_skills_dir) {
+                tracing::warn!(dir = %self.picobot_skills_dir.display(), error = %e, "Failed to create skills directory");
+            } else {
+                tracing::info!(dir = %self.picobot_skills_dir.display(), "Created skills directory");
+            }
+        }
+
         // Load from ~/.picobot/skills
         if self.picobot_skills_dir.exists() {
             let loaded = self.load_skills_from_dir(&self.picobot_skills_dir);