feat: add merge-request issue type for refinery processing

Add TypeMergeRequest as a valid issue type for tracking merge queue
entries. This enables the refinery to track merge requests with
structured metadata (branch, target, source_issue, worker, rig).

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

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

cmd/bd/create.go

@@ -392,7 +392,7 @@ 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)")
+	createCmd.Flags().StringP("type", "t", "task", "Issue type (bug|feature|task|epic|chore|merge-request)")
 	registerCommonIssueFlags(createCmd)
 	createCmd.Flags().StringSliceP("labels", "l", []string{}, "Labels (comma-separated)")
 	createCmd.Flags().StringSlice("label", []string{}, "Alias for --labels")