docs: add plan for graph expansion and optimization

[sandeepmanimala]

This PR introduces a PLAN.md file that outlines a roadmap for expanding and scaling the Multi-Omics Knowledge Graph.

Based on discussions with the user, this plan details two main phases:

1. Phase 1: Expanding Data & Dimensions
   • Integration of Epigenomics (DNA Methylation) to link genes to epigenetic states.
   • Integration of Metabolomics (HMDB) to map circulating metabolites to the microbiome and phenomics.
   • Integration of 3D Protein Structures (AlphaFold) to add structural context to existing proteins and pharmacological docking.
2. Phase 2: Performance & Scale Optimizations
   • Implementation of Apache Spark (PySpark) to handle massive ETL processes required for new billion-row datasets.
   • Refactoring ingestion to utilize the native Neo4j Admin Import tool over transactional Cypher imports.
   • Database tuning, B-tree/vector indexing, and transitioning to a clustered Neo4j Enterprise architecture.

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PR created automatically by Jules for task 6869368970008525253 started by @sandeepmanimala

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