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/validation/bead.go

@@ -32,11 +32,13 @@ func ParseIssueType(content string) (types.IssueType, error) {
 
 	// Validate issue type
 	validTypes := map[types.IssueType]bool{
-		types.TypeBug:     true,
-		types.TypeFeature: true,
-		types.TypeTask:    true,
-		types.TypeEpic:    true,
-		types.TypeChore:   true,
+		types.TypeBug:          true,
+		types.TypeFeature:      true,
+		types.TypeTask:         true,
+		types.TypeEpic:         true,
+		types.TypeChore:        true,
+		types.TypeMergeRequest: true,
+		types.TypeMolecule:     true,
 	}
 
 	if !validTypes[issueType] {