Add storage-specific AGENTS.md for sdk/storage subtree

[Copilot]

Adds sdk/storage/AGENTS.md to give AI agents (Copilot Coding Agent, etc.) storage-specific context that is too narrow for the root AGENTS.md.

What's in the file

• Module table — maps each azure-storage-* module to its Azure service
• 4 agent rules:
  • Do not edit generated code in implementation/ or models/ paths — fix upstream in TypeSpec/OpenAPI
  • Preserve API consistency (parameter names, types, option bag names, return types) across Blob, Queue, File, and Data Lake clients
  • Prefer existing patterns (RequestRetryOptions, ParallelTransferOptions, PagedIterable/PagedFlux, Reactor async) over new abstractions
  • No magic strings — use HttpHeaderName, service-specific *Constants classes, and *ServiceVersion enums
• Storage service semantics — per-service behavioral notes (AppendBlob overwrite safety, BlockBlob commit flow, Queue visibility timeout, File share hierarchy)
• Build/test commands — including Azurite startup and live-test invocation