feat: add roadmap issue template for planned features and improvements

.github/ISSUE_TEMPLATE/roadmap.yml

@@ -0,0 +1,18 @@
+name: 📌 Roadmap
+description: Planned feature or improvement (maintainer use)
+labels: ["type: feature", "source: maintainer"]
+
+body:
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      placeholder: What and why
+    validations:
+      required: true
+
+  - type: input
+    id: related
+    attributes:
+      label: Related issues
+      placeholder: "#123, #456"