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>

package-lock.json

@@ -12,10 +12,14 @@
       },
       "devDependencies": {
         "@types/node": "^25.5.2",
+        "openclaw": "file:/usr/lib/node_modules/openclaw",
         "typescript": "^5.6.3",
         "vitest": "^4.1.3"
       }
     },
+    "../../../../../usr/lib/node_modules/openclaw": {
+      "dev": true
+    },
     "node_modules/@emnapi/core": {
       "version": "1.9.1",
       "resolved": "https://registry.npmjs.org/@emnapi/core/-/core-1.9.1.tgz",
@@ -914,6 +918,10 @@
       ],
       "license": "MIT"
     },
+    "node_modules/openclaw": {
+      "resolved": "../../../../../usr/lib/node_modules/openclaw",
+      "link": true
+    },
     "node_modules/pathe": {
       "version": "2.0.3",
       "resolved": "https://registry.npmjs.org/pathe/-/pathe-2.0.3.tgz",