chore(repo): create shared utils package with toError helper

[zrosenbauer]

Summary

Multiple packages duplicate the instanceof Error → new Error(String(...)) coercion pattern inline. The @zpress/cli package already has a toError utility (packages/cli/src/lib/error.ts), but other packages (config, core) use inline variants.

Proposal

• Create a shared @zpress/utils package (or similar) with common helpers like toError
• Replace all inline instanceof Error coercion patterns across packages with the shared util
• Consolidate any other duplicated utility patterns discovered along the way

Locations

• packages/config/src/loader.ts:118
• packages/config/scripts/generate-schema.ts:53
• packages/core/src/banner/index.ts:184
• packages/cli/src/commands/diff.ts:349
• packages/cli/src/lib/error.ts (existing util to promote)