fix: support local models by making thought field optional in Plan model (#601)

- Make thought field optional in Plan model to fix Pydantic validation errors with local models
- Add Ollama configuration example to conf.yaml.example
- Update documentation to include local model support
- Improve planner prompt with better JSON format requirements

Fixes local model integration issues where models like qwen3:14b would fail
due to missing thought field in JSON output.

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

README.md

@@ -95,6 +95,7 @@ cp .env.example .env
 
 # Configure conf.yaml for your LLM model and API keys
 # Please refer to 'docs/configuration_guide.md' for more details
+# For local development, you can use Ollama or other local models
 cp conf.yaml.example conf.yaml
 
 # Install marp for ppt generation