Add typed workspace create-conflict mapping

[kauandotnet]

Summary

• add the typed mapCreateConflict hook required by Artifact Studio
• map only atomic create collisions while preserving replace and non-conflict failures
• keep optional and union-shaped AI tools provider-compatible without weakening runtime Zod validation
• include the existing minor Changeset for the next Storage alpha

Why

Artifact Studio already consumes this contract from local Storage commit 04d1707. The exact S3 capability change is merged upstream, but the published package cannot replace the sibling checkout until this contract is also on Storage main and released.

Compatibility

This is a clean application of 04d1707 onto current Storage main at a0ea392. It preserves the merged canonical ETag and workspace cursor behavior. It adds no new cursor mechanism, database migration, credential handling, or tenant-isolation change.

Supports nestm-dev/artifact-studio#22.

Validation

• pnpm run check
• pnpm run test — 236 unit tests passed; 55 E2E tests passed; 4 live-provider tests skipped
• pnpm run verify:pack
• pnpm run test:packed
• git diff --check origin/main...HEAD