fix(agent): add OpenRouter provider support and guard against unknown models

Use generic OPENROUTER_* env vars (OPENROUTER_API_KEY, OPENROUTER_BASE_URL,
OPENROUTER_MODEL) via the existing fallback mechanism instead of hardcoding
mappings to OPENAI_* vars. Also add a guard when resolveModel returns
undefined to throw a clear error instead of crashing on property access.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/runner.ts

@@ -177,6 +177,14 @@ export class Agent {
     const effectiveModel = resolvedModel ?? options.model ?? storedMeta?.model;
     let model = resolveModel({ ...options, provider: effectiveProvider, model: effectiveModel });
 
+    if (!model) {
+      throw new Error(
+        `Unknown model: provider="${effectiveProvider}", model="${effectiveModel}". ` +
+        `Check your LLM_PROVIDER and model env vars (e.g. OPENAI_MODEL). ` +
+        `For OpenRouter, use LLM_PROVIDER=openrouter.`,
+      );
+    }
+
     // Override base URL if provided via options or environment variable
     const baseUrl = resolveBaseUrl(model.provider, options.baseUrl);
     if (baseUrl) {