feat(opencode-zen): add OpenCode Zen (PAYG) provider with free-tier fingerprint, alias ocz

Multi-endpoint provider on https://opencode.ai/zen/v1 (openai / claude /
openai-responses transports mirroring opencode-go) with 71 models across
paid + free tiers. Free-tier fingerprint: opencode/1.18.x UA spoof,
ses_-session header, built-in tool quartet, forced stream. Usage endpoint
/zen/v1/usage wired into the dashboard.

open-sse/providers/registry/index.js

@@ -69,6 +69,7 @@ import p66 from "./ollama.js";
 import p123 from "./ollama-search.js";
 import p67 from "./openai.js";
 import p68 from "./opencode-go.js";
+import p68z from "./opencode-zen.js";
 import p69 from "./opencode.js";
 import p70 from "./openrouter.js";
 import p71 from "./perplexity-web.js";
@@ -194,6 +195,7 @@ export default [
   p123,
   p67,
   p68,
+  p68z,
   p69,
   p70,
   p71,