Certify OpenCode 1.18.18 shared-client contract

Record Gitea run 4055 as the contract clock for the pending 1.18.18
image. Shared-client only: no consumer promotion, no GitHub artifact
claim, rails persistence still unverified.

manifests/image-matrix.json

@@ -112,7 +112,19 @@
       "profiles": ["ruby-rest-sse"],
       "required_consumer_profiles": [],
       "certification_scope": "shared-client-contract-only",
-      "certification_status": "pending"
+      "certification_status": "certified",
+      "current_certification": {
+        "status": "certified",
+        "client_commit": "9277646a4bb2cf25a8384ffc140b154f49ea5766",
+        "rails_commit": "a9add2a7c1dd3eb978aa8b4ebf9ef7e111d1057f",
+        "checked_at": "2026-08-18T21:31:08Z",
+        "expected_text": "compat-ok",
+        "full_text": "compat-ok",
+        "llm_request_count": 1,
+        "workflow_run_id": "4055",
+        "workflow_head_sha": "88809aa98dc6e76976bf42ba9ab28e86f1aca39b",
+        "evidence": "evidence/2026-08-18-opencode-1.18.18-shared-client.json"
+      }
     }
   ],
   "host_canary": [