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/core/runtime.ts

@@ -42,10 +42,8 @@ import { verifySignature } from "../../../Yonexus.Protocol/src/crypto.js";
 import type { YonexusServerStore } from "./store.js";
 import { type ClientConnection, type ServerTransport } from "./transport.js";
 import { createPairingService, type PairingService } from "../services/pairing.js";
-import {
-  createDiscordNotificationService,
-  type DiscordNotificationService
-} from "../notifications/discord.js";
+import type { PairingNotificationService } from "../notifications/types.js";
+import { createNotificationService } from "../notifications/factory.js";
 import { safeErrorMessage } from "./logging.js";
 import type { ServerRuleRegistry } from "./rules.js";
 
@@ -53,7 +51,7 @@ export interface YonexusServerRuntimeOptions {
   config: YonexusServerConfig;
   store: YonexusServerStore;
   transport: ServerTransport;
-  notificationService?: DiscordNotificationService;
+  notificationService?: PairingNotificationService;
   ruleRegistry?: ServerRuleRegistry;
   onClientAuthenticated?: (identifier: string) => void;
   now?: () => number;
@@ -70,7 +68,7 @@ export class YonexusServerRuntime {
   private readonly now: () => number;
   private readonly registry: ServerRegistry;
   private readonly pairingService: PairingService;
-  private readonly notificationService: DiscordNotificationService;
+  private readonly notificationService: PairingNotificationService;
   private readonly sweepIntervalMs: number;
   private sweepTimer: NodeJS.Timeout | null = null;
   private started = false;
@@ -86,10 +84,7 @@ export class YonexusServerRuntime {
     this.pairingService = createPairingService({ now: this.now });
     this.notificationService =
       options.notificationService ??
-      createDiscordNotificationService({
-        botToken: options.config.notifyBotToken,
-        adminUserId: options.config.adminUserId
-      });
+      createNotificationService(options.config);
   }
 
   get state(): ServerLifecycleState {