Add an optional positional `<id>` argument to `nbd next` and `nbd ready`
that restricts results to the dependency subtree of the given ticket.
The scoping ticket itself is excluded from results.
- CLI: add `id: Option<String>` to `Commands::Next` and `Commands::Ready`
- Logic: build a `TicketGraph`, call `subtree()`, restrict candidate pool
- Tests: 4 new integration tests covering scoped next/ready and --filter
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Elijah Voigt