feat(agents): add isGeminiModel detection function with TDD

Detects Gemini models via:
- Provider prefixes: google/, google-vertex/
- GitHub Copilot: github-copilot/gemini-*
- Model name: gemini-* (for proxied providers like litellm)

Follows existing isGptModel pattern. All 16 tests pass.

src/agents/types.ts

@@ -80,6 +80,19 @@ export function isGptModel(model: string): boolean {
   return GPT_MODEL_PREFIXES.some((prefix) => modelName.startsWith(prefix))
 }
 
+const GEMINI_PROVIDERS = ["google/", "google-vertex/"]
+
+export function isGeminiModel(model: string): boolean {
+  if (GEMINI_PROVIDERS.some((prefix) => model.startsWith(prefix)))
+    return true
+
+  if (model.startsWith("github-copilot/") && extractModelName(model).toLowerCase().startsWith("gemini"))
+    return true
+
+  const modelName = extractModelName(model).toLowerCase()
+  return modelName.startsWith("gemini-")
+}
+
 export type BuiltinAgentName =
   | "sisyphus"
   | "hephaestus"