[#136] Vault → Write state.json atomically - #186

Resolves #136

Problem

state.json is written straight through the vault adapter, so a crash mid write can leave it torn; that file is the common ancestor every sync diffs against, and a torn read already falls back to an empty snapshot, which turns every remotely deleted file into a resurrection and every real divergence into a conflict copy

Why

• State corruption here is silent, it only ever surfaces later as confusing sync behaviour, never as an error at write time
• Reusing the existing install path means no new failure handling was invented, the same guarantee just gained a second caller

Greptile Summary

The PR makes persisted vault snapshots crash-safe by staging encoded state in a hidden sibling file and installing it through the existing replacement path.

• Reuses the vault writer's staged rename behavior for state.json.
• Adds adapter rename support to the store test fixture.
• Verifies a failed installation preserves the previous snapshot.

Confidence Score: 5/5

The PR appears safe to merge, with snapshot writes now preserving the prior state when installation fails.

The changed store path writes the complete encoded snapshot to a sibling temporary file before invoking the existing replacement logic, while production synchronization serializes store writes and the tests verify prior-state preservation on failure.

Important Files Changed

_{Reviews (1): Last reviewed commit: "Write state.json atomically" | Re-trigger Greptile}

Context used:

• Knowledge Base — Vault module