Add storage defaults implementation plan

[zzstoatzz]

this PR adds the OSS implementation plan for the Storage Defaults project.

It is intended to review and settle the OSS plan before more implementation work lands.

details

• documents the OSS target hierarchy and non-goals
• inventories the current OSS code paths involved in result-storage resolution
• proposes phased OSS delivery across OSS-7740, OSS-7741, and OSS-7739
• captures Cloud/workspace/account constraints only where they affect OSS design decisions
• identifies the OSS architectural questions that should be answered before more feature PRs

This PR does not change runtime behavior.