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.

tests/__baseline__/providers-baseline.json

@@ -762,6 +762,44 @@
       }
     ]
   },
+  "opencode-zen": {
+    "baseUrl": "https://opencode.ai/zen/v1/chat/completions",
+    "headers": {},
+    "usage": {
+      "url": "https://opencode.ai/zen/v1/usage"
+    },
+    "format": "openai",
+    "transports": [
+      {
+        "format": "openai",
+        "baseUrl": "https://opencode.ai/zen/v1/chat/completions",
+        "auth": {
+          "combined": true,
+          "header": "Authorization",
+          "scheme": "bearer"
+        }
+      },
+      {
+        "format": "claude",
+        "baseUrl": "https://opencode.ai/zen/v1/messages",
+        "auth": {
+          "combined": true,
+          "header": "x-api-key",
+          "scheme": "raw",
+          "anthropicVersion": true
+        }
+      },
+      {
+        "format": "openai-responses",
+        "baseUrl": "https://opencode.ai/zen/v1/responses",
+        "auth": {
+          "combined": true,
+          "header": "Authorization",
+          "scheme": "bearer"
+        }
+      }
+    ]
+  },
   "opencode": {
     "baseUrl": "https://opencode.ai",
     "headers": {