chore: move docker-chat-sandbox under templates/x (#23777)

Adds the experimental `docker-chat-sandbox` example template under
`examples/templates/x/`. It provisions a regular dev agent plus a
chat-designated agent that runs inside bubblewrap with a read-only root,
writable `/home/coder`, and outbound TCP restricted to the Coder
control-plane endpoint via `iptables`.

The chat agent still appears in dashboard and API responses, but the
template reserves it for chatd-managed sessions rather than normal user
interaction. `lint/examples` now walks nested template directories, so
experimental templates can live under `examples/templates/x/` without
treating `x/` itself as a template.

scripts/examplegen/main.go

@@ -49,17 +49,25 @@ func run(lint bool) error {
 
 	var paths []string
 	if lint {
-		files, err := fs.ReadDir(examplesFS, "templates")
+		err := fs.WalkDir(examplesFS, "templates", func(path string, d fs.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
+			if !d.IsDir() {
+				return nil
+			}
+			if path == "templates" {
+				return nil
+			}
+			if !isTemplateExampleDir(examplesFS, path) {
+				return nil
+			}
+			paths = append(paths, path)
+			return fs.SkipDir
+		})
 		if err != nil {
 			return err
 		}
-
-		for _, f := range files {
-			if !f.IsDir() {
-				continue
-			}
-			paths = append(paths, filepath.Join("templates", f.Name()))
-		}
 	} else {
 		for _, comment := range src.Comments {
 			for _, line := range comment.List {
@@ -102,6 +110,18 @@ func run(lint bool) error {
 	return enc.Encode(examples)
 }
 
+func isTemplateExampleDir(examplesFS fs.FS, name string) bool {
+	readmePath := path.Join(name, "README.md")
+	mainTFPath := path.Join(name, "main.tf")
+	if _, err := fs.Stat(examplesFS, readmePath); err != nil {
+		return false
+	}
+	if _, err := fs.Stat(examplesFS, mainTFPath); err != nil {
+		return false
+	}
+	return true
+}
+
 func parseTemplateExample(projectFS, examplesFS fs.FS, name string) (te *codersdk.TemplateExample, err error) {
 	var errs []error
 	defer func() {