v1.15.15

Engram v1.15.15 is a single-fix hotfix release: the legacy cloud-upgrade evaluator now correctly handles relation entities, unblocking `cloud upgrade` for projects that hit the regression introduced in #379.

## Highlights

- **Legacy relation upsert payloads are repaired during cloud upgrade**: `cloud upgrade` no longer blocks on `entity=relation op=upsert` mutations that lacked the fields added by #379. The legacy evaluator whitelists `SyncEntityRelation` and applies the full repair path before forwarding mutations to the cloud server.

## Contribution highlights

Thanks to the report and the precise reproduction that drove this fix:

- #383 came from a user blocked on `engram cloud upgrade doctor` and `engram sync --cloud` after upgrading to v1.15.13. The report pointed directly at the missing `SyncEntityRelation` case in the legacy evaluator and made the fix surgical.

## Fixes included

- `evaluateCloudUpgradeLegacyMutationTx` whitelists `SyncEntityRelation/upsert` and adds the full `case SyncEntityRelation:` repair branch, matching the path already used by the chunk canonicalizer and cloud server.

## Changelog

- `7af10b9` `fix(store): repair legacy relation upsert payloads` (closes #383)