feat(identity-wallet): implement PlcMonitor::check_all
Implements multi-identity monitoring that:
- Lists all managed identities from IdentityStore
- Calls check_for_changes for each identity sequentially
- Aggregates results into IdentityStatus list
- Returns complete state in one call, avoiding N+1 IPC calls
Verifies AC6.1-AC6.2 for multi-identity variant.