feat(providers): add four OpenAI-compatible aggregator providers (dahl, atria, agnes, bai)

Adds Dahl Inference, Atria Dawn, Agnes AI, and B.AI following the existing
registry pattern. Each uses DefaultExecutor with no custom translator needed.

open-sse/providers/registry/agnes.js

@@ -0,0 +1,29 @@
+export default {
+  id: "agnes",
+  priority: 120,
+  alias: "agnes",
+  aliases: [
+    "agnes-ai",
+  ],
+  uiAlias: "agnes",
+  display: {
+    name: "Agnes AI",
+    icon: "auto_awesome",
+    color: "#7C3AED",
+    textIcon: "AG",
+    website: "https://agnes-ai.com",
+    notice: {
+      text: "OpenAI-compatible gateway from Agnes AI, offering free API credits on sign-up. Accepts a bearer token or an x-api-key header.",
+      apiKeyUrl: "https://platform.agnes-ai.com",
+    },
+  },
+  category: "freeTier",
+  authType: "apikey",
+  transport: {
+    baseUrl: "https://apihub.agnes-ai.com/v1/chat/completions",
+    validateUrl: "https://apihub.agnes-ai.com/v1/models",
+  },
+  // No model ids could be verified without a key, so discovery is left to the
+  // live endpoint and any id is accepted through passthroughModels.
+  passthroughModels: true,
+};