feat: upgrade MiniMax from M2.5 to M2.7 and expand to more agents/categories

- Upgrade minimax-m2.5 → minimax-m2.7 (latest model) across all agents and categories
- Replace minimax-m2.5-free with minimax-m2.7-highspeed (optimized speed variant)
- Expand MiniMax fallback coverage to atlas, sisyphus-junior, writing, and unspecified-low
- Add isMiniMaxModel() detection function in types.ts for model family detection
- Update all tests (58 passing) and documentation

src/agents/types.ts

@@ -91,6 +91,11 @@ export function isGpt5_3CodexModel(model: string): boolean {
 
 const GEMINI_PROVIDERS = ["google/", "google-vertex/"];
 
+export function isMiniMaxModel(model: string): boolean {
+  const modelName = extractModelName(model).toLowerCase();
+  return modelName.includes("minimax");
+}
+
 export function isGeminiModel(model: string): boolean {
   if (GEMINI_PROVIDERS.some((prefix) => model.startsWith(prefix))) return true;