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.test.ts

@@ -336,8 +336,8 @@ describe("resolveModelWithFallback", () => {
       expect(logSpy).toHaveBeenCalledWith("No available model found in fallback chain, falling through to system default")
     })
 
-    test("returns system default when availableModels is empty", () => {
-      // #given
+    test("uses first fallback entry when availableModels is empty (no cache scenario)", () => {
+      // #given - empty availableModels simulates CI environment without model cache
       const input: ExtendedModelResolutionInput = {
         fallbackChain: [
           { providers: ["anthropic"], model: "claude-opus-4-5" },
@@ -349,9 +349,9 @@ describe("resolveModelWithFallback", () => {
       // #when
       const result = resolveModelWithFallback(input)
 
-      // #then
-      expect(result.model).toBe("google/gemini-3-pro")
-      expect(result.source).toBe("system-default")
+      // #then - should use first fallback entry, not system default
+      expect(result.model).toBe("anthropic/claude-opus-4-5")
+      expect(result.source).toBe("provider-fallback")
     })
 
     test("returns system default when fallbackChain is not provided", () => {

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}`