fix(mol): substitute title/desc vars in root molecule bead

When pouring a formula with `title` and `desc` variables defined,
the root molecule's title and description now use {{title}} and
{{desc}} placeholders that get substituted during pour.

Previously, the root was always assigned the formula name and static
description, ignoring these common variables. Child beads correctly
substituted variables, but the root did not.

Fixes #852

cmd/bd/cook.go

@@ -443,11 +443,25 @@ func cookFormulaToSubgraph(f *formula.Formula, protoID string) (*TemplateSubgrap
 	var issues []*types.Issue
 	var deps []*types.Dependency
 
+	// Determine root title: use {{title}} placeholder if the variable is defined,
+	// otherwise fall back to formula name (GH#852)
+	rootTitle := f.Formula
+	if _, hasTitle := f.Vars["title"]; hasTitle {
+		rootTitle = "{{title}}"
+	}
+
+	// Determine root description: use {{desc}} placeholder if the variable is defined,
+	// otherwise fall back to formula description (GH#852)
+	rootDesc := f.Description
+	if _, hasDesc := f.Vars["desc"]; hasDesc {
+		rootDesc = "{{desc}}"
+	}
+
 	// Create root proto epic
 	rootIssue := &types.Issue{
 		ID:          protoID,
-		Title:       f.Formula, // Title is the original formula name
-		Description: f.Description,
+		Title:       rootTitle,
+		Description: rootDesc,
 		Status:      types.StatusOpen,
 		Priority:    2,
 		IssueType:   types.TypeEpic,
@@ -783,11 +797,25 @@ func cookFormula(ctx context.Context, s storage.Storage, f *formula.Formula, pro
 	var deps []*types.Dependency
 	var labels []struct{ issueID, label string }
 
+	// Determine root title: use {{title}} placeholder if the variable is defined,
+	// otherwise fall back to formula name (GH#852)
+	rootTitle := f.Formula
+	if _, hasTitle := f.Vars["title"]; hasTitle {
+		rootTitle = "{{title}}"
+	}
+
+	// Determine root description: use {{desc}} placeholder if the variable is defined,
+	// otherwise fall back to formula description (GH#852)
+	rootDesc := f.Description
+	if _, hasDesc := f.Vars["desc"]; hasDesc {
+		rootDesc = "{{desc}}"
+	}
+
 	// Create root proto epic using provided protoID (may include prefix)
 	rootIssue := &types.Issue{
 		ID:          protoID,
-		Title:       f.Formula, // Title is the original formula name
-		Description: f.Description,
+		Title:       rootTitle,
+		Description: rootDesc,
 		Status:      types.StatusOpen,
 		Priority:    2,
 		IssueType:   types.TypeEpic,