feat: skills delete — 交互式删除用户 Skill

新增 ias skills delete:
  扫描 skills/ 目录，列出所有用户 Skill
  MultiSelect TUI 选择要删除的 Skill
  确认后 remove_dir_all 物理删除

src/cli.rs

@@ -77,4 +77,6 @@ pub enum SkillsAction {
     Import,
     /// 列出可导入的 Skill
     List,
+    /// 交互式删除用户 Skill
+    Delete,
 }

src/main.rs

@@ -814,5 +814,62 @@ async fn cmd_skills(action: SkillsAction) {
             println!("\n完成：导入 {imported} 个，跳过 {skipped} 个");
             println!("重启 iAs 后生效");
         }
+        SkillsAction::Delete => {
+            let skills_dir = std::path::PathBuf::from("skills");
+            if !skills_dir.exists() {
+                println!("skills 目录不存在，没有可删除的 Skill");
+                return;
+            }
+
+            let mut entries: Vec<(String, String)> = Vec::new();
+            if let Ok(dir) = std::fs::read_dir(&skills_dir) {
+                for entry in dir.flatten() {
+                    let path = entry.path();
+                    if path.is_dir() && path.join("SKILL.md").exists() {
+                        let name = path.file_name().unwrap_or_default().to_string_lossy().to_string();
+                        let desc = std::fs::read_to_string(path.join("SKILL.md"))
+                            .ok()
+                            .and_then(|s| s.lines().nth(0).map(|l| l.trim_start_matches("# ").to_string()))
+                            .unwrap_or_default();
+                        entries.push((name, desc));
+                    }
+                }
+            }
+            entries.sort_by(|a, b| a.0.cmp(&b.0));
+
+            if entries.is_empty() {
+                println!("skills 目录下没有可删除的 Skill");
+                return;
+            }
+
+            let items: Vec<String> = entries.iter()
+                .map(|(name, desc)| format!("{} — {}", name, desc))
+                .collect();
+
+            println!("选择要删除的 Skill（空格选择，回车确认，q 退出）:");
+            let selection = MultiSelect::with_theme(&ColorfulTheme::default())
+                .items(&items)
+                .interact_on(&Term::stderr())
+                .unwrap_or_default();
+
+            if selection.is_empty() {
+                println!("未选择任何 Skill");
+                return;
+            }
+
+            let mut deleted = 0;
+            for idx in selection {
+                let (name, _) = &entries[idx];
+                let path = skills_dir.join(name);
+                match std::fs::remove_dir_all(&path) {
+                    Ok(_) => {
+                        println!("  ✅ 已删除 {name}");
+                        deleted += 1;
+                    }
+                    Err(e) => println!("  ❌ 删除 {name} 失败: {e}"),
+                }
+            }
+            println!("\n完成：删除 {deleted} 个");
+        }
     }
 }