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/discord.ts

@@ -5,15 +5,14 @@
  */
 
 import type { PairingRequest } from "../services/pairing.js";
+import type { PairingNotificationService } from "./types.js";
 import { redactPairingCode, safeErrorMessage } from "../core/logging.js";
 
-export interface DiscordNotificationService {
-  /**
-   * Send a pairing code notification to the admin user.
-   * @returns Whether the notification was sent successfully
-   */
-  sendPairingNotification(request: PairingRequest): Promise<boolean>;
-}
+/**
+ * Back-compat alias. The Discord notifier is one provider implementing
+ * the neutral {@link PairingNotificationService} contract.
+ */
+export type DiscordNotificationService = PairingNotificationService;
 
 export interface DiscordNotificationConfig {
   botToken: string;