Design optional centralized snapshot registry (out of scope for core library)

[cursor]

Summary

Snapshot storage design keeps object storage / centralized registry out of scope. Teams may still want shared approved baselines across repos. Capture the design as an ADR or proposal without implementing a database in this package by default.

Acceptance criteria

• Short ADR/proposal: auth, retention, content-addressed snapshots, relationship to Git baselines + provenance sidecars
• Explicit non-goals for the core tool-semantics install
• Recommendation: stay with Git baselines vs optional companion service
• Link from docs/superpowers/specs/2026-08-13-snapshot-storage-design.md

Non-goals for this issue

• Implementing the registry service in-repo unless a follow-up issue is filed after design approval