feat(cli): redesign model fallback with native cross-fallback and OpenAI separation

- Add OpenAI/ChatGPT as separate subscription option (--openai flag)
- Implement native tier cross-fallback (Claude → OpenAI → Gemini)
- Change fallback order: Native → OpenCode Zen → GitHub Copilot → Z.ai
- Add explore agent special logic: max20 → haiku, else → grok-code
- Add critical warning when Claude is not configured
- Add tests for ChatGPT-only and explore agent cases

src/cli/types.ts

@@ -4,6 +4,7 @@ export type BooleanArg = "no" | "yes"
 export interface InstallArgs {
   tui: boolean
   claude?: ClaudeSubscription
+  openai?: BooleanArg
   gemini?: BooleanArg
   copilot?: BooleanArg
   opencodeZen?: BooleanArg
@@ -14,6 +15,7 @@ export interface InstallArgs {
 export interface InstallConfig {
   hasClaude: boolean
   isMax20: boolean
+  hasOpenAI: boolean
   hasGemini: boolean
   hasCopilot: boolean
   hasOpencodeZen: boolean
@@ -30,6 +32,7 @@ export interface DetectedConfig {
   isInstalled: boolean
   hasClaude: boolean
   isMax20: boolean
+  hasOpenAI: boolean
   hasGemini: boolean
   hasCopilot: boolean
   hasOpencodeZen: boolean