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

skills/sample-command/SKILL.md

@@ -0,0 +1,17 @@
+---
+name: fetch-page
+description: Fetch and summarize a web page
+argument-hint: <url>
+user-invocable: true
+---
+
+# Fetch Page
+
+Fetch the content of a URL and produce a concise summary.
+
+## Process
+
+1. Fetch the URL passed as `$1` using the `web_fetch` tool
+2. Summarize the main content in 3–5 sentences, preserving key facts and figures
+3. List any notable links or resources mentioned in the page
+4. If the URL is inaccessible, report the error and suggest checking the URL format