Implements intelligent model selection based on available providers.
Priority order: Native > Copilot > OpenCode Zen > Z.ai > Ultimate Fallback.
- Add model-fallback.ts with MODEL_CATALOG and capability-based resolution
- Extend InstallConfig with hasOpencodeZen and hasZaiCodingPlan
- Add TUI prompts and CLI args for new provider options
- Z.ai gets Librarian priority (zai-coding-plan/glm-4.7)
🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)
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