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>

internal/types/types.go

@@ -233,12 +233,13 @@ const (
 	TypeChore        IssueType = "chore"
 	TypeMessage      IssueType = "message"       // Ephemeral communication between workers
 	TypeMergeRequest IssueType = "merge-request" // Merge queue entry for refinery processing
+	TypeMolecule     IssueType = "molecule"      // Composable workflow template
 )
 
 // IsValid checks if the issue type value is valid
 func (t IssueType) IsValid() bool {
 	switch t {
-	case TypeBug, TypeFeature, TypeTask, TypeEpic, TypeChore, TypeMessage, TypeMergeRequest:
+	case TypeBug, TypeFeature, TypeTask, TypeEpic, TypeChore, TypeMessage, TypeMergeRequest, TypeMolecule:
 		return true
 	}
 	return false