#215 feat(core): normalize writeback receipt outcomes

Summary

• add normalized writeback status states, including first-class no_receipt
• add WritebackError extending RelayfileWritebackError for existing catch compatibility
• re-export the normalizer and error from adapter-core/vfs-client for runtime consumers

Why

Supports AgentWorkforce/cloud#2291 W6/W2 follow-up work: workforce can wrap writeJsonFile once and cloud can map normalized state/path into terminal cause taxonomy.

Notes

Opened from the already-pushed branch so downstream workforce work can reference a concrete PR instead of a local-only branch.