feat: selectable pairing-notify provider (discord optional, add fabric)

Pairing-code delivery was hardwired to Discord DM (notifyBotToken +
adminUserId required). Make the provider config-selectable.

- core/config.ts: add notifyProvider ("discord"|"fabric", default
  "discord" for back-compat); discord fields required only for discord;
  add fabric block (centerApiBase/apiKey/guildNodeId/channelId) required
  only for fabric
- notifications/types.ts: neutral PairingNotificationService interface
  (DiscordNotificationService kept as back-compat alias)
- notifications/fabric.ts: post the pairing message to a Fabric channel
  (agent/login -> guild token -> POST messages); self-contained, no
  Fabric plugin dependency
- notifications/factory.ts: select provider from config
- core/runtime.ts: wire via factory
- openclaw.plugin.json: notifyProvider enum + fabric object; drop
  notifyBotToken/adminUserId from required (conditional in code)
- tests: fabric notifier + provider-selection config (80 passing)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/notifications/types.ts

@@ -0,0 +1,18 @@
+/**
+ * Yonexus Server - Pairing notification provider interface.
+ *
+ * A provider delivers the out-of-band pairing code to a human admin.
+ * Implementations: Discord DM (`./discord.ts`), Fabric channel
+ * (`./fabric.ts`). The provider is selected by config (see
+ * `core/config.ts` `notifyProvider`).
+ */
+
+import type { PairingRequest } from "../services/pairing.js";
+
+export interface PairingNotificationService {
+  /**
+   * Send a pairing code notification to the admin.
+   * @returns Whether the notification was sent successfully.
+   */
+  sendPairingNotification(request: PairingRequest): Promise<boolean>;
+}