docs: MEAI migration Phase 0 discovery notes

[IanFrelinger]

Summary

Phase 0 (read-only discovery) for the Microsoft.Extensions.AI governed pipeline migration. Adds docs/meai-migration-notes.md mapping current model invocation, sanitization, audit, policy, RAG, and DI seams to proposed MEAI concepts.

Changes

• New docs/meai-migration-notes.md with concrete types + paths for all six discovery items
• Mapping table: existing types → IChatClient / DelegatingChatClient / IEmbeddingGenerator / VectorStore
• Documented gaps: TFM is net8.0 today (plan asks net9), local offline is LLamaSharp not ONNX Runtime, no Bedrock/MEAI yet, dual IModel vs NCR paths

Testing

• Documentation-only change; no code/tests required for Phase 0 acceptance

Testing strategy (blast radius)

Change type	Minimum proof
Docs only	Review of notes against source (discovery)

• Not applicable — docs only
• Documentation updated

Checklist

• Documentation updated
• No code changes (Phase 0 is read-only aside from the notes deliverable)
• Breaking changes are documented — N/A

Follow-up

Phase 1 (Nexo.AI.Pipeline, Ollama/LLamaSharp IChatClient, Nexo:UseMeaiPipeline flag default off) will land in a separate PR per the migration plan.