Update code review docs: custom instructions read from head branch (#62248)

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Copilot-Session: afad84f8-25e0-43c6-ba7b-2506a82bed5a

Author: 3 people

content/copilot/concepts/agents/code-review.md

@@ -160,6 +160,8 @@ When {% data variables.copilot.copilot_code-review_short %} is enabled for these
 
 If your repository includes agent skills, {% data variables.copilot.copilot_code-review_short %} can automatically use relevant skills when reviewing a pull request, extending {% data variables.product.prodname_copilot_short %} beyond its built-in analysis.
 
+{% data reusables.copilot.code-review.custom-instructions-branch %}
+
 For setup details, see [AUTOTITLE](/copilot/how-tos/copilot-on-github/customize-copilot/customize-cloud-agent/add-skills).
 
 ### MCP servers

data/reusables/copilot/code-review/custom-instructions-branch.md

@@ -1 +1 @@
-When reviewing a pull request, {% data variables.product.prodname_copilot_short %} uses the custom instructions in the base branch of the pull request. For example, if your pull request seeks to merge `my-feature-branch` into `main`, {% data variables.product.prodname_copilot_short %} will use the custom instructions in `main`.
+When reviewing a pull request, {% data variables.product.prodname_copilot_short %} reads repository custom instructions, agent instructions, and agent skills from the head branch (the branch with your changes), not the base branch. For example, when merging `my-feature-branch` into `main`, {% data variables.product.prodname_copilot_short %} uses the instructions and skills in `my-feature-branch`, so you can test changes to them in the same pull request without merging them first.