Add repository instruction file

Authored by: Abhijit Gadgil <agadgil@progress.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: agadgil-progress

.github/instructions/exclude-submodules.instructions.md

@@ -0,0 +1,10 @@
+PATH: .github/instructions/exclude-submodules.instructions.md
+applyTo:
+  - "**/*"
+exclude:
+  - "vendor/**"
+  - "third_party/**"
+  - "**/.git/modules/**"
+  - paths listed in .gitmodules
+rules:
+  - "Do not propose or apply edits in excluded paths; treat them as read-only references."