harness-providers + app: dedicated opencode (Zen) provider

OpenAiProvider gains a custom id and a chat-only mode; `OpenAiProvider::opencode`
builds an OpenCode Zen provider (id `opencode`, defaults to Zen's gateway, always
chat-completions so even gpt-*/o* model ids route correctly). Registered from a
`providers.opencode` config block / OPENCODE_API_KEY env, so it coexists with a
real `openai` provider. Models are referenced as `opencode/<model>`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/harness-app/src/lib.rs

@@ -147,6 +147,19 @@ impl EngineHandle {
             };
             providers.register(Arc::new(provider));
         }
+        // OpenCode Zen: OpenAI-compatible chat gateway under its own `opencode` provider id,
+        // so it coexists with a real `openai` provider. `base_url` defaults to Zen's endpoint.
+        if let Some(key) = config
+            .providers
+            .get("opencode")
+            .and_then(|p| p.api_key.clone())
+        {
+            let base_url = config
+                .providers
+                .get("opencode")
+                .and_then(|p| p.base_url.clone());
+            providers.register(Arc::new(OpenAiProvider::opencode(key, base_url)));
+        }
 
         Self::build(cwd, store, config, providers)
     }