Integrate with git-bug format for interop

Investigate interoperability with git-bug's JSON-based format. git-bug stores issues as a DAG of operations in refs/bugs/. A bridge or converter that can translate between git-issue's commit-trailer format and git-bug's operation log would allow users of either tool to collaborate. This is separate from the GitHub/GitLab bridges and focuses on local Git-native interop between the two tools.