Dolt backend drops metadata set via bd update --metadata

[turian]

Summary

The Dolt backend silently drops JSON metadata set via bd update --metadata. The field is accepted by the CLI but not persisted — subsequent bd export output contains no metadata field.

Reproduction

bd init --prefix test --quiet
id=$(bd create --title "Test" --type task --silent)
bd update "$id" --metadata '{"component":"auth","risk":"high"}'
bd export  # metadata field missing from output

Expected behavior

The exported JSONL should include "metadata":{"component":"auth","risk":"high"} on the issue.

Actual behavior

The metadata field is absent from the export output entirely.

Context

• Discovered via differential regression test (TestMetadataRoundTrip in tests/regression/)
• v0.49.6 (SQLite backend) correctly preserves metadata through update and export
• Current main (Dolt backend) drops it silently
• Not a normalization artifact — confirmed by inspecting raw export output from a fresh candidate build

Classification

Data integrity regression: a field set via CLI is silently lost.

[turian]

Partially fixed by bfb5def (SELECT projection). Remaining fix (UPDATE normalization in transaction.go) available in PR #1910.