Adds a new `backlog` Status variant for tickets that are created but
intentionally deferred. Backlog tickets are excluded from `nbd list`,
`nbd ready`, and `nbd next` by default, but unlike `done` and `closed`
they do not count as resolved for dependency purposes — a ticket whose
dependency is `backlog` remains blocked.
Visible via `--all` or `--filter status=backlog`.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Implements TicketGraph<'a> with build(), roots(), subtree(),
to_json_value(), and to_subtree_json_value(). Tracks both forward
(dependencies) and reverse (dependents) edges. Cycle-safe DFS via
visited set. No new crate dependencies. 14 unit tests covering empty
graph, chains, branching, cycles, dangling refs, and JSON output.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Elijah Voigt