OpenClaw session tool visibility hardening and Telegram webhook secret fallback

OpenClaw is a personal AI assistant. Prior to version 2026.2.15, in some shared-agent deployments, OpenClaw session tools (`sessions_list`, `sessions_history`, `sessions_send`) allowed broader session targeting than some operators intended. This is primarily a configuration/visibility-scoping issue in multi-user environments where peers are not equally trusted. In Telegram webhook mode, monitor startup also did not fall back to per-account `webhookSecret` when only the account-level secret was configured. In shared-agent, multi-user, less-trusted environments: session-tool access could expose transcript content across peer sessions. In single-agent or trusted environments, practical impact is limited. In Telegram webhook mode, account-level secret wiring could be missed unless an explicit monitor webhook secret override was provided. Version 2026.2.15 fixes the issue.