Add flexible --format flag to 'bd list' command supporting:
- Built-in presets: 'digraph' (basic 'from to' format) and 'dot' (Graphviz)
- Custom Go templates for dependency output
- Template variables: IssueID, DependsOnID, Type, Issue, Dependency
This enables graph analysis with tools like golang.org/x/tools/cmd/digraph while allowing users to customize output format for their specific needs.
Examples:
bd list --format=digraph | digraph nodes
bd list --format=dot | dot -Tsvg -o deps.svg
bd list --format='{{.IssueID}} -> {{.DependsOnID}} [{{.Type}}]'
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