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/openclaw.plugin.json

@@ -16,12 +16,24 @@
         "type": "array",
         "items": { "type": "string" }
       },
+      "notifyProvider": { "type": "string", "enum": ["discord", "fabric"] },
       "notifyBotToken": { "type": "string" },
       "adminUserId": { "type": "string" },
+      "fabric": {
+        "type": "object",
+        "additionalProperties": false,
+        "properties": {
+          "centerApiBase": { "type": "string" },
+          "apiKey": { "type": "string" },
+          "guildNodeId": { "type": "string" },
+          "channelId": { "type": "string" }
+        },
+        "required": ["centerApiBase", "apiKey", "guildNodeId", "channelId"]
+      },
       "listenHost": { "type": "string" },
       "listenPort": { "type": "number" },
       "publicWsUrl": { "type": "string" }
     },
-    "required": ["notifyBotToken", "adminUserId", "listenPort"]
+    "required": ["listenPort"]
   }
 }