feat(plugin-audit): sys_comment gains a writer-stamped object_name + record_id pair with a backfill, and the comment-thread read gate leafs over it (ADR-0139 E7)

feat(plugin-audit): sys_comment gains a writer-stamped object_name + record_id pair with a backfill, and the comment-thread read gate leafs over it (ADR-0139 E7) #15432