StorageProvider: Add ConfigCollection facade or remove config from CollectionEntityMap

[diberry]

Context

Flight's Phase 2 review found that config is declared in CollectionEntityMap (collection-map.ts:31) but has no corresponding ConfigCollection class in collections.ts and no state.config property in SquadState.

Options

1. Add a stub ConfigCollection — read-only access to .squad/config.json via StorageProvider
2. Remove config from CollectionEntityMap — if config is not intended as a state-layer concern

Branch

Work should be done on diberry/sa-phase1-interface

Severity

Should-fix (non-blocking for Phase 3 CLI migration)

[diberry]

ConfigCollection added with get()/update()/exists(), wired to SquadState, 5 tests. Shipped on diberry/sa-phase1-interface.