Add 'molecule' as valid issue type

Molecules are composable workflow templates used by Gas Town (gt) for
defining multi-step processes like bootstrap, engineer-in-box, etc.

Changes:
- Add TypeMolecule constant to internal/types/types.go
- Add molecule to IsValid() switch case
- Add molecule and merge-request to validation/bead.go validTypes map
- Update help text in create.go, show.go, list.go
- Add test cases for molecule and merge-request types

Closes gt-qn4l

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

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

cmd/bd/list.go

@@ -547,7 +547,7 @@ func init() {
 	listCmd.Flags().StringP("status", "s", "", "Filter by status (open, in_progress, blocked, 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)")
+	listCmd.Flags().StringP("type", "t", "", "Filter by type (bug, feature, task, epic, chore, merge-request, molecule)")
 	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)")