Bug: sync.md uses wrong gh-sub-issue syntax

[shtirlitsDva]

references/sync.md line 68 has a dead comment that never runs:

# gh sub-issue create --parent $epic_number ...

Two problems:

1. This is just a comment — when use_subissues=true, nothing actually links tasks to the epic
2. gh sub-issue create doesn't support --body-file or --json, so it can't replace the existing gh issue create call

Fix: create with gh issue create (keeps --body-file/--json), then link with gh sub-issue add:

if [ "$use_subissues" = true ]; then
  gh sub-issue add "$epic_number" "$task_number" --repo "$REPO"
fi