feat(ai-service): implement complete Orchestrator generation pipeline for T3.4 [AC-AISVC-01, AC-AISVC-02]
- Integrate Memory, ContextMerger, Retriever, LLMClient, ConfidenceCalculator
- Implement 8-step generation pipeline:
1. Load local history from Memory
2. Merge with external history (dedup + truncate)
3. RAG retrieval (optional)
4. Build prompt with context and evidence
5. LLM generation
6. Calculate confidence
7. Save messages to Memory
8. Return ChatResponse
- Add GenerationContext dataclass for tracking intermediate results
- Implement fallback response mechanism for error handling
- Add 21 unit tests for OrchestratorService
- All 138 tests passing
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