feat(providers): add System One (Jev) decision endpoint

New /v1/systemone pass-through route for Jev decision models (jev-1.13,
jev-1.13-free) on OpenCode Zen and the free lane. Follows the media-route
pattern: systemoneConfig in the registry drives URL/headers, the handler
mirrors the embeddings account-fallback + usage flow, and the dashboard
gains a System One media-provider kind. No chat-pipeline changes.

Co-Authored-By: Claude Code <noreply@anthropic.com>

open-sse/providers/registry/opencode.js

@@ -28,7 +28,16 @@ export default {
     { id: "muse-spark-1.2-contributor-free", name: "Muse Spark 1.2 Contributor Free", targetFormat: "openai-responses" },
     { id: "muse-spark-1.3-contributor-free", name: "Muse Spark 1.3 Contributor Free", targetFormat: "openai-responses" },
     { id: "union-alpha", name: "Union Alpha Free", targetFormat: "claude" },
+    { id: "jev-1.13-free", name: "Jev 1.13 Free", kind: "decision" },
   ],
+  serviceKinds: ["llm", "systemone"],
+  systemoneConfig: {
+    baseUrl: "https://opencode.ai/zen/v1/systemone",
+    headers: {
+      "x-opencode-client": "desktop",
+      "User-Agent": "opencode/1.18.31",
+    },
+  },
   modelsFetcher: { url: "https://opencode.ai/zen/v1/models", type: "opencode-free" },
   passthroughModels: true,
 };