feat: Generic file storage abstraction layer

[falfaddaghi]

Summary

Introduces a pluggable StorageProvider abstraction layer for all server-side file I/O in Nexus Workflow Studio. The current tight coupling to node:fs/promises in workspace, brain, snapshot, and collaboration modules is replaced by a single interface with a LocalFilesystemProvider default. Adding new storage backends (S3, Azure Blob, SharePoint) will require only implementing the interface and registering the provider — no changes to any consumer.

Implementation plan: docs/tasks/feature-generic-file-storage-b8dbb4a6/plan-feature-generic-file-storage-b8dbb4a6.md

ADW tracking ID: b8dbb4a6

Checklist

• Define StorageProvider interface and supporting types (StorageMetadata, StorageProviderType) in src/lib/storage/types.ts
• Create storage configuration with NEXUS_STORAGE_PROVIDER and NEXUS_STORAGE_ROOT env var support
• Implement LocalFilesystemProvider wrapping current fs behavior with path traversal guard
• Build provider factory with caching, reset, and registerStorageProvider() for extensibility
• Create barrel export at src/lib/storage/index.ts
• Write unit tests for LocalFilesystemProvider (all interface methods + edge cases)
• Write unit tests for factory (selection, caching, reset, custom registration)
• Migrate workspace/server.ts to use StorageProvider (remove direct fs imports)
• Migrate workspace/snapshots.ts to use StorageProvider
• Migrate brain/server.ts to use StorageProvider
• Migrate collaboration/object-store.ts to accept a StorageProvider
• Update config files to document new env vars
• Create E2E test specification
• Validate: bun run typecheck, bun run lint, bun run build all pass

Key Changes

• New src/lib/storage/ module with types.ts, local-provider.ts, factory.ts, config.ts, index.ts
• All server-side consumers (workspace/server.ts, workspace/snapshots.ts, brain/server.ts, collaboration/object-store.ts) migrated off direct node:fs/promises imports
• registerStorageProvider() API enables third-party storage backends with zero core changes
• writeAtomic() method preserves crash-safe temp-file-then-rename behavior used by snapshots and collab state
• Binary data (Uint8Array) supported via readBytes()/writeBytes() for collaboration state