fix(model-resolver): use first fallback entry when model cache unavailable

When availableModels is empty (no cache in CI), use the first entry
from fallbackChain directly instead of falling back to systemDefault.
This ensures categories and agents use their configured models even
when the model cache file doesn't exist.

Fixes:
- model-resolution check returning 'warn' instead of 'pass' in CI
- DEFAULT_CATEGORIES not being used when no cache available
- Unstable agent detection failing (models falling back to non-gemini)

src/shared/model-resolver.ts

@@ -53,6 +53,15 @@ export function resolveModelWithFallback(
 
 	// Step 2: Provider fallback chain (with availability check)
 	if (fallbackChain && fallbackChain.length > 0) {
+		// If availableModels is empty (no cache), use first fallback entry directly without availability check
+		if (availableModels.size === 0) {
+			const firstEntry = fallbackChain[0]
+			const firstProvider = firstEntry.providers[0]
+			const model = `${firstProvider}/${firstEntry.model}`
+			log("Model resolved via fallback chain (no cache, using first entry)", { provider: firstProvider, model: firstEntry.model, variant: firstEntry.variant })
+			return { model, source: "provider-fallback", variant: firstEntry.variant }
+		}
+
 		for (const entry of fallbackChain) {
 			for (const provider of entry.providers) {
 				const fullModel = `${provider}/${entry.model}`