Extend copilot-instructions.md (openvinotoolkit#3332)

## Description
From
openvinotoolkit#2707 (comment)
## Checklist:
- [x] This PR follows GenAI Contributing guidelines. <!-- Always follow
https://github.com/openvinotoolkit/openvino.genai?tab=contributing-ov-file#contributing.
If there are deviations, explain what and why. -->
- N/A Tests have been updated or added to cover the new code. <!--
Specify exactly which tests were added or updated. If the change isn't
maintenance related, update the tests at
https://github.com/openvinotoolkit/openvino.genai/tree/master/tests or
explain in the description why the tests don't need an update. -->
- N/A This PR fully addresses the ticket. <!--- If not, explain clearly
what is covered and what is not. If follow-up pull requests are needed,
specify in the description. -->
- N/A I have made corresponding changes to the documentation. <!-- Run
github.com/\<username>/openvino.genai/actions/workflows/deploy_gh_pages.yml
on your fork with your branch as a parameter to deploy a test version
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---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: Wovchena

.github/copilot-instructions.md

@@ -33,11 +33,12 @@ When analyzing a Pull Request, follow this protocol:
 12. Make sure the function names are descriptive.
 13. Check for variables with different names but similar meaning or aliasing.
 14. Avoid duplicate code. Ensure that common functionality is extracted into reusable functions or utilities.
-15. Avoid pronouns in comments and names to make the statements concise.
-16. Unused functions and constructors aren't allowed except for in `debug_utils.hpp`.
-17. `debug_utils.hpp` must never be included.
-18. Assumptions on user behalf aren't allowed. For example, the implementation shouldn't adjust config values silently or with a warning; it should throw an exception instead.
-19. Samples:
+15. When initial container values are known upfront, prefer initializer-list / brace-initialization over constructing an empty container and immediately inserting values.
+16. Avoid pronouns in comments and names to make the statements concise.
+17. Unused functions and constructors aren't allowed except for in `debug_utils.hpp`.
+18. `debug_utils.hpp` must never be included.
+19. Assumptions on the user's behalf aren't allowed. For example, the implementation shouldn't adjust config values silently or with a warning; it should throw an exception instead.
+20. Samples:
     * Avoid adding new samples unless there is a strong, clearly justified reason.
     * Keep command‑line arguments in samples minimal.
     * Ensure new samples have corresponding tests.