Introduce local store auth summary API

[dmerand]

Summary

Introduce a local auth summary API without changing shopify store list behavior yet.

Parent and child

PR	Role
#7708	BP-only store list
#7709	local auth summary API
#7714	local source + auto fallback

Scope

• add StoredStoreAuthSummary
• add listStoredStoreAuthSummaries() under services/store/auth
• keep token-bearing session buckets as the current persistence source of truth
• keep conf traversal and bucket cleanup in session-store.ts
• re-export the summary API from services/store/auth/index.ts
• fix the stale store list command test assertion after the BP-only trim

Why this split exists

The next PR needs one narrow question answered first:

• what locally stored store auth exists on this machine?

This PR answers that without taking on product behavior:

• no --source local
• no BP→local fallback
• no persistence migration

That keeps the behavior PR focused on command policy and output, not on storage traversal.

Boundary this PR is trying to establish

• persistence details stay in session-store.ts
• callers see StoredStoreAuthSummary[]
• callers do not need token-bearing session buckets or conf layout knowledge