Configure OMX Notifications

Unified and only entry point for notification setup.

• Native integrations (first-class): Discord, Telegram, Slack
• Generic extensibility integrations: custom_webhook_command, custom_cli_command

Standalone configure skills (configure-discord, configure-telegram, configure-slack, configure-openclaw) are removed.

Step 1: Inspect Current State

CONFIG_FILE="$HOME/.codex/.omx-config.json"