fix: preserve migration headers after failed commit

[sean0407]

Fixes #119.

Summary

- await writeCurrentMigration(..., body)
+ await writeCurrentMigration(..., serializeMigration(body, headers))

When commit fails, parseMigrationText has separated migration headers from the SQL body. Restoring only the body drops the --! Message: header from current.sql. This change serializes the parsed headers with the body before restoring it.

Open PR #262 separately restores --!include directives in this rollback path; this change covers the distinct loss of migration headers.

Evidence

• Before: A failing commit with --! Message: test-migration restored current.sql without the header.
• After: The regression test verifies the restored file exactly matches the original and the failed committed migration is removed.

Validation

• yarn test:only __tests__/commit.test.ts — 14 passed.
• yarn prettier:check — passed.

Merge Danger

Door: Two-way

Blast Radius: Low

The change applies only to restoring the current migration after a failed commit.

[benjie]

Thanks for spotting the issue! Rather than recreating the text with headers, I've just fixed the issue: use the original text that we read.