feat: 权限档位体系（三档 plan/workspace/full + 四桥 from_session_id）

L2 核心改动：sessions 表补 permission_mode / permission_enforcement 两列
（sqlite + pg 同步），三桥 lib/permission-mode.js 翻译档位到平台原生配置，
homeagent advisory 模式提示词告知模型实际强制力。四桥全部携带 from_session_id
供 relay 去重与会话回溯。

FromHuman / ToHuman 判据已加入心跳 payload 与 notify/mail.go。

plugins/homeagent-mail-bridge/tools.go

@@ -69,8 +69,9 @@ func (p *Plugin) handleReadMail(args map[string]interface{}) (interface{}, error
 	if len(data.Mail.Attachments) > 0 {
 		fmt.Fprintf(&sb, "附件:\n")
 		for _, a := range data.Mail.Attachments {
-			fmt.Fprintf(&sb, "  - %s (%.1fKB, id=%s)\n", a.Filename, float64(a.SizeBytes)/1024, a.AttachmentID)
+			fmt.Fprintf(&sb, "  - %s (%s, id=%s)\n", a.Filename, formatSize(int64(a.SizeBytes)), a.AttachmentID)
 		}
+		sb.WriteString("  用 download_attachment 取回（传 attachment_id 与 save_path）\n")
 	}
 	fmt.Fprintf(&sb, "\n%s\n", data.Mail.Body)