Fix opencode model flag format and add agent execution logging

OpenCode expects --model in provider/model format (e.g.
opencode-go/minimax-m2.7) but buildAgentCommand was passing just the
model ID. This caused opencode to silently fail with no output across
all iterations.

Also add logging for agent execution start/finish with exit code,
output size, and duration to help diagnose future failures.

apps/harness/src/lib/agents.ts

@@ -118,7 +118,14 @@ export function buildAgentCommand(config: AgentConfig, prompt: string, workDir:
   const args = [runtime.cliCommand];
 
   if (runtime.headlessFlag) args.push(runtime.headlessFlag);
-  if (runtime.modelFlag && config.modelId) args.push(runtime.modelFlag, config.modelId);
+
+  if (runtime.modelFlag && config.modelId) {
+    // OpenCode expects --model in provider/model format
+    const modelArg = config.runtime === "opencode" && config.provider
+      ? `${config.provider}/${config.modelId}`
+      : config.modelId;
+    args.push(runtime.modelFlag, modelArg);
+  }
 
   if (runtime.promptFlag) {
     args.push(runtime.promptFlag, prompt);

apps/harness/src/lib/orchestrator.ts

@@ -308,6 +308,8 @@ async function runIteration(
     priorIterations,
   });
 
+  console.log(`[orchestrator] Running agent ${task.spec.agentId} for task ${task.id} iter ${n} in ${workDir}`);
+
   const execResult = await executeAgent({
     agentId: task.spec.agentId,
     prompt,
@@ -315,6 +317,8 @@ async function runIteration(
     signal: currentAbort?.signal,
   });
 
+  console.log(`[orchestrator] Agent finished: exit=${execResult.exitCode} killed=${execResult.killed} stdout=${execResult.stdout.length}b stderr=${execResult.stderr.length}b duration=${execResult.durationMs}ms`);
+
   if (execResult.killed && currentAbort?.signal.aborted) {
     await updateIteration(task.id, n, {
       status: "failed",