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/cli/model-fallback.ts

@@ -55,7 +55,7 @@ export function generateModelConfig(config: InstallConfig): GeneratedOmoConfig {
   for (const [role, req] of Object.entries(CLI_AGENT_MODEL_REQUIREMENTS)) {
     if (role === "librarian") {
       if (avail.opencodeGo) {
-        agents[role] = { model: "opencode-go/minimax-m2.5" }
+        agents[role] = { model: "opencode-go/minimax-m2.7" }
       } else if (avail.zai) {
         agents[role] = { model: ZAI_MODEL }
       }
@@ -68,7 +68,7 @@ export function generateModelConfig(config: InstallConfig): GeneratedOmoConfig {
       } else if (avail.opencodeZen) {
         agents[role] = { model: "opencode/claude-haiku-4-5" }
       } else if (avail.opencodeGo) {
-        agents[role] = { model: "opencode-go/minimax-m2.5" }
+        agents[role] = { model: "opencode-go/minimax-m2.7" }
       } else if (avail.copilot) {
         agents[role] = { model: "github-copilot/gpt-5-mini" }
       } else {