Provide cherry-pick guidance as a skill instead of instructions

Per review feedback, move the cherry-pick convention out of .github/copilot-instructions.md (which is injected into every prompt) and into an on-demand skill at .github/skills/cherry-pick/SKILL.md. The skill is auto-discovered from the repo and only loaded when relevant, keeping it out of unrelated prompts.

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

Author: alsepkow

.github/copilot-instructions.md

@@ -0,0 +1,23 @@
+---
+name: cherry-pick
+description: Cherry-pick a commit onto another branch and open the PR. Use when the user asks to cherry-pick or backport a commit or PR to a release branch.
+---
+
+# Cherry-picking commits
+
+Use this when asked to cherry-pick or backport a change.
+
+1. Identify the source commit SHA to cherry-pick. If the source PR was
+   squash-merged, the single squashed commit on the target branch is what you
+   cherry-pick.
+2. Create a topic branch off the destination branch.
+3. Apply the change with `git cherry-pick -x <sha>` so the commit message
+   records the source commit in git's standard format:
+
+   ```
+   (cherry picked from commit <sha>)
+   ```
+
+   Keep the cherry-pick commit message as git generates it by default.
+4. Open the PR with the title prefixed `[Cherry-Pick]`, followed by the
+   original commit subject.