feat: add commit message guidelines

- Introduce guidelines for writing commit messages.
- Emphasize the use of Conventional Commit format.
- Include tips for brevity and clarity in descriptions.

.copilot-commit-message-instructions.md

@@ -0,0 +1,7 @@
+Use Conventional Commit format.
+Avoid overly verbose descriptions or unnecessary details.
+Start with a short sentence in imperative form, no more than 50 characters long.
+Then leave an empty line and continue with a more detailed explanation.
+Write only one sentence for the first part, and two or three sentences at most for the detailed explanation.
+Keep message as short as possible.
+Use bullet points for multiple changes.