feat: refactor prompt registry and preview pipeline

[starboyate]

Summary

• refactor the Java prompt builders around shared prompt builder support and a registry-based override pipeline
• add aligned default and expanded prompt preview flows so preview output comes from the real prompt templates
• wire PromptRegistry through the memory builder, LLM runtime services, and Spring Boot auto-configuration while removing the unused summaryPrompt path

Test Plan

• mvn test
• mvn -pl memind-core -Dtest=PromptTemplateTest,PromptRegistryTest,InMemoryPromptRegistryTest,PromptDefaultsTest,PromptPreviewTest,MemoryItemUnifiedPromptsTest,SelfVerificationPromptsTest,TypedQueryExpandPromptsTest,InsightGroupPromptsTest,InsightLeafPromptsTest,BranchAggregationPromptsTest,RootSynthesisPromptsTest,InteractionGuideSynthesisPromptsTest test