fix(model-resolver): skip fallback chain when model availability cannot be verified

When model cache is empty, the fallback chain resolution was blindly
trusting connected providers without verifying if the model actually
exists. This caused errors when a provider (e.g., opencode) was marked
as connected but didn't have the requested model (e.g., claude-haiku-4-5).

Now skips fallback chain entirely when model cache is unavailable and
falls through to system default, letting OpenCode handle the resolution.

src/shared/model-resolver.test.ts

@@ -356,8 +356,10 @@ describe("resolveModelWithFallback", () => {
       cacheSpy.mockRestore()
     })
 
-    test("uses connected provider when availableModels empty but connected providers cache exists", () => {
+    test("skips fallback chain when availableModels empty even if connected providers cache exists", () => {
       // #given - model cache missing but connected-providers cache exists
+      // This scenario caused bugs: provider is connected but may not have the model available
+      // Fix: When we can't verify model availability, skip fallback chain entirely
       const cacheSpy = spyOn(connectedProvidersCache, "readConnectedProvidersCache").mockReturnValue(["openai", "google"])
       const input: ExtendedModelResolutionInput = {
         fallbackChain: [
@@ -370,9 +372,32 @@ describe("resolveModelWithFallback", () => {
       // #when
       const result = resolveModelWithFallback(input)
 
-      // #then - should use openai (second provider) since anthropic not in connected cache
-      expect(result!.model).toBe("openai/claude-opus-4-5")
-      expect(result!.source).toBe("provider-fallback")
+      // #then - should fall through to system default (NOT use connected provider blindly)
+      expect(result!.model).toBe("google/gemini-3-pro")
+      expect(result!.source).toBe("system-default")
+      cacheSpy.mockRestore()
+    })
+
+    test("prevents selecting model from provider that may not have it (bug reproduction)", () => {
+      // #given - user removed anthropic oauth, has quotio, but explore agent fallback has opencode
+      // opencode may be "connected" but doesn't have claude-haiku-4-5
+      const cacheSpy = spyOn(connectedProvidersCache, "readConnectedProvidersCache").mockReturnValue(["quotio", "opencode"])
+      const input: ExtendedModelResolutionInput = {
+        fallbackChain: [
+          { providers: ["anthropic", "opencode"], model: "claude-haiku-4-5" },
+        ],
+        availableModels: new Set(), // no model cache available
+        systemDefaultModel: "quotio/claude-opus-4-5-20251101",
+      }
+
+      // #when
+      const result = resolveModelWithFallback(input)
+
+      // #then - should NOT return opencode/claude-haiku-4-5 (model may not exist)
+      // should fall through to system default which user has configured
+      expect(result!.model).toBe("quotio/claude-opus-4-5-20251101")
+      expect(result!.source).toBe("system-default")
+      expect(result!.model).not.toBe("opencode/claude-haiku-4-5")
       cacheSpy.mockRestore()
     })