Make BuildTree read-only: never write to stack.json from graph/view

BuildTree was silently rewriting orphaned branches to trunk and saving.
A read-only command (graph, view) should not mutate state.
Now only modifies in-memory config for display, never calls Save().

Author: mubbie

internal/stack/tree.go

@@ -53,20 +53,15 @@ func BuildTree() *BranchStack {
 		}
 	}
 
-	// Clean stale parent references (parent branch was deleted)
-	changed := false
+	// For display purposes, treat branches whose parent was deleted as parented to trunk.
+	// Do NOT save this - graph is read-only.
 	for name, meta := range cfg.Branches {
 		if _, exists := branchSHAs[meta.Parent]; !exists {
-			// Parent no longer exists, default to trunk
 			if meta.Parent != mainBranch {
 				cfg.Branches[name] = &BranchMeta{Parent: mainBranch, ParentHead: meta.ParentHead}
-				changed = true
 			}
 		}
 	}
-	if changed {
-		_ = cfg.Save()
-	}
 
 	// No self-healing: only show branches explicitly in stack.json.
 	// Use `gx stack` to add branches to the stack.