v1.15.7

## Summary

- Fixed legacy local sync imports that stalled on orphan prompt/observation session dependencies.
- Allows old malformed `.engram/chunks` exports to recover data instead of failing forever with a foreign key error.

## Details

- Local `engram sync --import` now scans manifest chunks for real session upserts before deciding a dependency is truly missing.
- When a missing session is absent from both the local DB and every chunk, import synthesizes a deterministic recovered session stub with directory `(recovered-missing-session)`.
- Existing cross-chunk dependency-safe behavior is preserved: real session chunks still win over recovered stubs.

## Validation

- `go test ./internal/sync -run 'TestLocalImport(RecoversLegacyChunkWithMissingSessionStub|DependencySafeAcrossChunksRegardlessManifestOrder|OrdersExplicitMutationsAndDirectArraysSafely)' -count=1 -v`
- `go test ./internal/sync ./internal/store -count=1`
- `go test ./cmd/engram ./internal/cloud/cloudserver ./internal/cloud/cloudstore ./internal/cloud/remote ./internal/sync ./internal/store -count=1`
- `go test ./... -count=1`
- `git diff --check`