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/show.go

@@ -1338,7 +1338,7 @@ func init() {
 	updateCmd.Flags().StringP("status", "s", "", "New status")
 	registerPriorityFlag(updateCmd, "")
 	updateCmd.Flags().String("title", "", "New title")
-	updateCmd.Flags().StringP("type", "t", "", "New type (bug|feature|task|epic|chore|merge-request)")
+	updateCmd.Flags().StringP("type", "t", "", "New type (bug|feature|task|epic|chore|merge-request|molecule)")
 	registerCommonIssueFlags(updateCmd)
 	updateCmd.Flags().String("notes", "", "Additional notes")
 	updateCmd.Flags().String("acceptance-criteria", "", "DEPRECATED: use --acceptance")