/**
 * Built-in notification channels.
 *
 * Webhook and Slack are always registered. Delivery uses (in order):
 *   1. per-notification `metadata.webhookUrl` / `metadata.slackWebhookUrl`
 *   2. the matching `NOTIFICATIONS_*` env URL (workspace default)
 * If neither is set, that channel no-ops for the emission.
 *
 * Extra channels can be registered at any time via
 * `registerNotificationChannel()` from a server plugin.
 *
 * NOTIFICATIONS_WEBHOOK_URL  → default POST JSON URL when metadata omits one.
 *                              Supports `${keys.NAME}` substitution — the raw
 *                              value never enters the agent context.
 * NOTIFICATIONS_WEBHOOK_AUTH → optional `Authorization` header value (also
 *                              supports `${keys.NAME}`).
 * NOTIFICATIONS_SLACK_WEBHOOK_URL
 *                            → default Slack incoming webhook when metadata
 *                              omits `slackWebhookUrl`.
 *                              Supports `${keys.NAME}` substitution.
 * NOTIFICATIONS_EMAIL_CHANNEL=1
 *                            → enable the built-in email channel for
 *                              per-notification recipients.
 * NOTIFICATIONS_EMAIL_RECIPIENTS
 *                            → comma-separated fallback recipients for email
 *                              notifications that do not pass
 *                              `metadata.emailRecipients`.
 */
export declare function registerBuiltinNotificationChannels(): void;
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