refactor: add another check for dotdirs when iterating the config tree (#2279)

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## What this PR does / why we need it:

Normally, dotdirs are skipped when parsing the config already, but in
case of delayed loading via `LoadSubTree`, we can bypass this and still
end up loading files from dotdirs. This PR adds an extra check to
exclude dotdirs when visiting the config tree. This fixes a bug in
Catalyst.

## Does this PR introduce a user-facing change?
<!--
If no, just write "no" in the block below.
If yes, please explain the change and update documentation and the
CHANGELOG.md file accordingly.
-->
```
no
```

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Small, localized traversal refactor that only affects which tree nodes
are visited; main risk is unintentionally excluding previously-visible
stacks if callers relied on dotdir contents.
> 
> **Overview**
> Updates config tree traversal used by `Stacks()`, `StacksByPaths`,
`StacksByTagsFilters`, and `StackByID` to skip dot-directories during
DFS, even when nodes were introduced via `LoadSubTree`.
> 
> This refactors the internal helper from `stacks()` to `visit()` and
applies an explicit `Skip(name)` check while walking `Tree.Children`,
preventing hidden directories from contributing stacks to query results.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
db0ac00. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: soerenmartius

config/config.go

@@ -281,7 +281,7 @@ func (root *Root) StacksByPaths(base project.Path, relpaths ...string) List[*Tre
 			logger.Warn().Msgf("path %s not found in configuration", path.String())
 			continue
 		}
-		stacks = append(stacks, node.stacks((*Tree).IsStack)...)
+		stacks = append(stacks, node.visit((*Tree).IsStack)...)
 	}
 
 	sort.Sort(stacks)
@@ -295,7 +295,7 @@ func (root *Root) StacksByTagsFilters(filters []string) (project.Paths, error) {
 	if err != nil {
 		return nil, err
 	}
-	return root.tree.stacks(func(tree *Tree) bool {
+	return root.tree.visit(func(tree *Tree) bool {
 		if !hasFilter || !tree.IsStack() {
 			return false
 		}
@@ -305,7 +305,7 @@ func (root *Root) StacksByTagsFilters(filters []string) (project.Paths, error) {
 
 // StackByID returns the stack with the given ID.
 func (root *Root) StackByID(id string) (*Stack, bool, error) {
-	stacks := root.tree.stacks(func(tree *Tree) bool {
+	stacks := root.tree.visit(func(tree *Tree) bool {
 		return tree.IsStack() && tree.Node.Stack.ID == id
 	})
 	if len(stacks) == 0 {
@@ -477,7 +477,7 @@ func (tree *Tree) Stack() (*Stack, error) {
 // Stacks returns the stack nodes from the tree.
 // The search algorithm is a Deep-First-Search (DFS).
 func (tree *Tree) Stacks() List[*Tree] {
-	stacks := tree.stacks((*Tree).IsStack)
+	stacks := tree.visit((*Tree).IsStack)
 	sort.Sort(stacks)
 	return stacks
 }
@@ -517,15 +517,17 @@ func (tree *Tree) TerragruntModule() (*tg.Module, error) {
 	return nil, nil
 }
 
-func (tree *Tree) stacks(cond func(*Tree) bool) List[*Tree] {
-	var stacks List[*Tree]
+func (tree *Tree) visit(cond func(*Tree) bool) List[*Tree] {
+	var r List[*Tree]
 	if cond(tree) {
-		stacks = append(stacks, tree)
+		r = append(r, tree)
 	}
-	for _, children := range tree.Children {
-		stacks = append(stacks, children.stacks(cond)...)
+	for name, child := range tree.Children {
+		if !Skip(name) {
+			r = append(r, child.visit(cond)...)
+		}
 	}
-	return stacks
+	return r
 }
 
 // Lookup a node from the tree using a filesystem query path.