refactor: 删除外置 skills 系统，全部工具转为内置

删除:
- resources/skills/ (9个外置 skill, bash/python/node 脚本)
- src/tools/registry.rs (SkillRegistry 加载/执行)
- src/tools/parser.rs (SKILL.md 解析)
- src/skills_importer.rs (skills 导入)
- cli.rs Skills 子命令
- main.rs cmd_skills / global_skills_dir

保留:
- src/tools/builtin.rs (内置工具注册 + 审批流程)
- src/tools/weather.rs (天气查询)
- src/tools/skill.rs (精简为 SkillSpec/RiskLevel/SkillResult/ExecutionContext)
- src/tools/approval.rs (审批管理器)

main.rs 简化: 移除 SkillRegistry 加载, executor 只走 BuiltinRegistry

src/llm/provider.rs

@@ -1,6 +1,6 @@
 use crate::llm::types::{ConversationConfig, Message, StreamChunk};
-use serde::Deserialize;
 use async_trait::async_trait;
+use serde::Deserialize;
 use std::sync::Arc;
 use tokio::sync::mpsc;
 
@@ -33,29 +33,6 @@ pub fn create_provider(_config: &ConversationConfig) -> Result<BoxedProvider, St
 
 // ─── 内部：SSE 解析工具 ───
 
-/// 从字节流中解析 SSE 行
-pub(crate) fn parse_sse_line(line: &str) -> Option<(String, String)> {
-    // 支持两种格式:
-    //   data: {...}
-    //   event: ...
-    let line = line.trim();
-    if line.is_empty() || line.starts_with(':') {
-        return None;
-    }
-
-    if let Some(pos) = line.find(": ") {
-        let field = line[..pos].trim().to_string();
-        let value = line[pos + 2..].trim_start().to_string();
-        Some((field, value))
-    } else if let Some(pos) = line.find(':') {
-        let field = line[..pos].trim().to_string();
-        let value = line[pos + 1..].trim_start().to_string();
-        Some((field, value))
-    } else {
-        None
-    }
-}
-
 /// 流式块解析结果
 pub(crate) enum ParsedChunk {
     Text(String),
@@ -71,9 +48,7 @@ pub(crate) enum ParsedChunk {
 }
 
 /// 从 JSON body 中解析 DeepSeek/OpenAI 流式 delta
-pub(crate) fn parse_chat_chunk(
-    line: &str,
-) -> Option<ParsedChunk> {
+pub(crate) fn parse_chat_chunk(line: &str) -> Option<ParsedChunk> {
     if !line.starts_with("data: ") {
         return None;
     }
@@ -144,7 +119,9 @@ pub(crate) fn parse_chat_chunk(
         if let Some(tool_calls) = &choice.delta.tool_calls {
             for tc in tool_calls {
                 let idx = tc.index.unwrap_or(0);
-                let args = tc.function.as_ref()
+                let args = tc
+                    .function
+                    .as_ref()
                     .and_then(|f| f.arguments.clone())
                     .unwrap_or_default();
                 let name = tc.function.as_ref().and_then(|f| f.name.clone());