Split fastly_storage.rs + config store trait (read-only)

[aram356]

Description

Split fastly_storage.rs into storage/config_store.rs, storage/secret_store.rs, storage/api_client.rs. Implement PlatformConfigStore read path backed by Fastly SDK. Write stubs return PlatformError::NotImplemented.

Done when

• File split done; read path wired
• Write stubs return error (not unimplemented!())
• Per-PR gates pass

Blocked by

PR 2

References

• docs/internal/EDGEZERO_MIGRATION.md — Phase 1, PR 3
• Part of EdgeZero migration #480