feat: implement selfbot workspace submodule

- Add `discord.js-selfbot-v13` as a git submodule in `vendor/`
- Update `pnpm-workspace.yaml` to include the new submodule path
- Modify `.gitmodules` to track the submodule repository
- Change `discord.js-selfbot-v13` dependency in `package.json` to use `workspace:*`
- Create implementation and design documents for the submodule integration

package.json

@@ -27,7 +27,7 @@
     "@types/pg": "^8.20.0",
     "@vitejs/plugin-react": "^6.0.2",
     "better-sqlite3": "^12.10.0",
-    "discord.js-selfbot-v13": "^3.7.1",
+    "discord.js-selfbot-v13": "workspace:*",
     "dotenv": "^17.4.2",
     "drizzle-orm": "^0.45.2",
     "express": "^5.2.1",