Add shared spaces design spec

[Intina47]

Motivation

• Create a clear functional specification for team-shared notebooks and spaces to guide implementation and UX decisions.
• Ensure roles, sharing flows, and auditing requirements are defined to meet security and collaboration needs.
• Provide a single reference document for product and engineering to agree on scope and non-goals.

Description

• Add docs/shared-spaces.md containing an overview, goals, non-goals, core concepts, and role definitions (owner/editor/viewer).
• Define sharing models including direct invitations and link-sharing options with policy details and sharing flows.
• Specify membership/access rules, audit log events and requirements, and security/compliance considerations including token revocation and retention.
• Include open questions for implementation decisions such as audit log visibility and default link-sharing settings.

Testing

• No automated tests were run because this is a documentation-only change.

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Codex Task