fix: 完善所有渠道安装指引，修复文档错误

- 小红书: 添加 Docker 安装命令 (xpzouying/xiaohongshu-mcp)，替换模糊的 'install xiaohongshu-mcp'
- Exa: 修正文档错误（实际免费无需 API Key，之前写要 Key）
- CLI: install 命令输出添加小红书 Docker 安装指引
- channel: xiaohongshu.py 所有错误信息添加完整安装步骤
- install.md: 添加小红书配置段落
- README: 修正小红书 GitHub 链接，Exa 描述改为免费无需 Key

Fixes user-reported issue: 安装后不知道怎么装小红书 MCP

agent_reach/cli.py

@@ -433,7 +433,10 @@ def _install_mcporter():
                 )
                 print("  ✅ XiaoHongShu MCP auto-detected and configured")
             except Exception:
-                print("  ⬜ XiaoHongShu MCP not detected (optional — install xiaohongshu-mcp for XHS support)")
+                print("  ⬜ XiaoHongShu MCP not detected (optional)")
+                print("     Install: docker run -d --name xiaohongshu-mcp -p 18060:18060 xpzouying/xiaohongshu-mcp")
+                print("     Then:    mcporter config add xiaohongshu http://localhost:18060/mcp")
+                print("     Repo:    https://github.com/xpzouying/xiaohongshu-mcp")
     except Exception:
         pass