Summary

• Remove the "+ 添加自定义 Agent" button from the Local Agents page header
• Remove the "自定义" section (heading, inline editor, agent list, empty state) from the bottom of the page
• Clean up all now-unused state, callbacks, imports, and related dead code

Test plan

• Open Settings → Agents → 本地 Agents tab: confirm no "添加自定义 Agent" button appears
• Confirm the "自定义" section heading and empty-state text are gone
• Verify detected agents (Gemini CLI, Claude Code, etc.) still render correctly