* Add opencode as an AI generator option
Support to use OpenCode
(https://opencode.ai) instead of Claude Code for client generation.
- Adds --generator flag (claude|opencode, default: claude)
- OpenCode uses its own configured default model (--model is ignored)
- mimc doctor checks for either claude or opencode on PATH
- Minimal diff: one if/else branch in generate(), one new CLI flag
* docs: clean up comments
* Restore AI generator check in mimc doctor
