…vert prompts to English

- Convert defaultSummarizationPrompt from Chinese to English with structured
  Markdown format (Role → Task → Rules → Output Format) matching agent prompt style
- Add summarization_provider field to [context] config section, allowing compact
  summarization to use an independent LLM provider from global.llm.providers
- Extend ContextCompactConfig, compact.Config, and ConfigFrom with
  SummarizationProvider field
- Create independent summaryEngine in app.go when summarization_provider is set,
  falling back to main engine for backward compatibility
- Convert getSummarizationPrompt() in conductor.go from Chinese to English
- Export GetProvider in config package and add NewLoggingEngine factory in llm package
- Update all NewConductorAgent call sites in tests to pass nil summaryEngine