fix: surface stored metadata in recall results

[christopherkarani]

Closes #53.

Summary:

• Surface persisted frame metadata on search/recall result types (SearchResponse.Result, MemorySearchHit, RAGContext.Item).
• Thread metadata through unified search and RAG assembly so callers can recover app-specific IDs from context.items.
• Update the README and DocC guides to document the structured-data pattern.
• Add a regression test covering metadata propagation through MemoryOrchestrator.recall.

Verification:

• swift test --filter memoryOrchestratorRecallSurfacesStoredMetadata
• swift test --filter memoryOrchestratorSessionTaggingAndChunkMetadataPersist

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