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/factory.ts

@@ -0,0 +1,30 @@
+/**
+ * Yonexus Server - Pairing notification provider factory.
+ *
+ * Selects the notification provider from config (`notifyProvider`).
+ * Discord is the default for backward compatibility.
+ */
+
+import type { YonexusServerConfig } from "../core/config.js";
+import type { PairingNotificationService } from "./types.js";
+import { createDiscordNotificationService } from "./discord.js";
+import { createFabricNotificationService } from "./fabric.js";
+
+export function createNotificationService(
+  config: YonexusServerConfig
+): PairingNotificationService {
+  if (config.notifyProvider === "fabric") {
+    if (!config.fabric) {
+      throw new Error(
+        'Yonexus.Server: notifyProvider is "fabric" but fabric config is missing'
+      );
+    }
+    return createFabricNotificationService(config.fabric);
+  }
+
+  // Default / "discord"
+  return createDiscordNotificationService({
+    botToken: config.notifyBotToken ?? "",
+    adminUserId: config.adminUserId ?? ""
+  });
+}