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/create.go

@@ -499,7 +499,7 @@ func init() {
 	createCmd.Flags().String("title", "", "Issue title (alternative to positional argument)")
 	createCmd.Flags().Bool("silent", false, "Output only the issue ID (for scripting)")
 	registerPriorityFlag(createCmd, "2")
-	createCmd.Flags().StringP("type", "t", "task", "Issue type (bug|feature|task|epic|chore|merge-request|molecule|gate|agent|role)")
+	createCmd.Flags().StringP("type", "t", "task", "Issue type (bug|feature|task|epic|chore|merge-request|molecule|gate|agent|role|convoy)")
 	registerCommonIssueFlags(createCmd)
 	createCmd.Flags().StringSliceP("labels", "l", []string{}, "Labels (comma-separated)")
 	createCmd.Flags().StringSlice("label", []string{}, "Alias for --labels")