feat: add 'convoy' issue type with reactive completion (bd-hj0s)

- Add TypeConvoy to issue types for cross-project tracking
- Implement reactive completion: when all tracked issues close,
  convoy auto-closes with reason "All tracked issues completed"
- Uses 'tracks' dependency type (non-blocking, cross-prefix capable)
- Update help text for --type flag in list/create commands
- Add test for convoy reactive completion behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: beads/crew/dave
Rig: beads
Role: crew

cmd/bd/list.go

@@ -912,7 +912,7 @@ func init() {
 	listCmd.Flags().StringP("status", "s", "", "Filter by status (open, in_progress, blocked, deferred, closed)")
 	registerPriorityFlag(listCmd, "")
 	listCmd.Flags().StringP("assignee", "a", "", "Filter by assignee")
-	listCmd.Flags().StringP("type", "t", "", "Filter by type (bug, feature, task, epic, chore, merge-request, molecule, gate)")
+	listCmd.Flags().StringP("type", "t", "", "Filter by type (bug, feature, task, epic, chore, merge-request, molecule, gate, convoy)")
 	listCmd.Flags().StringSliceP("label", "l", []string{}, "Filter by labels (AND: must have ALL). Can combine with --label-any")
 	listCmd.Flags().StringSlice("label-any", []string{}, "Filter by labels (OR: must have AT LEAST ONE). Can combine with --label")
 	listCmd.Flags().String("title", "", "Filter by title text (case-insensitive substring match)")