Add RAG + Validation upgrade proposal (documentation)

[delisha02]

Motivation

• Provide a roadmap to upgrade the drafting pipeline to a legally grounded, auditable, and citation-aware system with higher practical quality.
• Address current gaps including missing retrieval in generation, isolated research RAG, lack of clause-level tagging/traceability, and limited validation capabilities.
• Establish a clear default of non-agentic RAG for routine drafting with a gated agentic mode for complex cases to balance latency, cost, and determinism.

Description

• Adds docs/rag_upgrade_proposal.md which specifies a recommended target architecture, including hybrid retrieval, clause-by-clause generation, a legal validation engine, citation traceability, and export formats.
• Defines priority workstreams P0/P1/P2 with concrete technical changes such as integrating retrieval into generate_document, implementing get_hybrid_retriever(...), adding clause schemas and per-clause provenance, and a deterministic validator with LLM-assisted fallbacks.
• Lists API and data-model additions including response fields for content, citations[], validation_report, clause_trace[], and persistent JSON fields like retrieval_sources.
• Includes rollout strategy, evaluation metrics to track (e.g., citation precision@k, missing clause rate, hallucination rate), risk controls, and minimum controls before full agentic rollout.

Testing

• This is a documentation-only change and no automated tests were run for this PR.

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Codex Task