[backport checklist] Add when/why guidance to checklist comment

[mrodm]

Proposed commit message

Add a sentence to the backport checklist comment explaining the core
decision rule ("backport if it fixes existing behavior; leave new
behavior on main") and linking to the Package Backports wiki page for
full context. Developers seeing the checklist for the first time now
have enough information to decide whether to tick a branch without
needing to search for documentation separately.

Author's Checklist

• Confirm the new sentence renders correctly in a GitHub PR comment preview.

How to test this PR locally

Run the backports test suite:

go test ./dev/backports/...

To eyeball the rendered output, create a minimal artifact file and run mage renderBackportChecklist:

cat <<EOF > pr-packages-test.json
{"number": 1, "packages": ["aws"]}
EOF

mage renderBackportChecklist ./pr-packages-test.json <<< ""

Check that the new guidance sentence and wiki link appear between the intro line and the first package section.

Example of the resulting checklist comment (from mrodm/integrations#78):

Backport branches

[!IMPORTANT]
Only branches for packages touched by this PR's current diff are shown.
This comment is updated automatically on each push — manual edits will be overwritten.

Tick the branches you want to backport to. PRs will be created automatically on merge, or when you update this checklist after merge.

Backport a change when it fixes behavior a branch already has; leave new behavior on main. See when and why to backport if you are unsure.

kubernetes

• backport-kubernetes-1.83
• backport-kubernetes-1.62
• backport-kubernetes-1.39

---

[!TIP]
If a branch above is no longer required, set archived: true in its entry in .backports.yml to stop it appearing here.
If the branch has a known end-of-life date, prefer maintained_until: "YYYY-MM-DD" — it will be excluded automatically once that date passes.

Related issues

• Relates Improve package backport tooling for routine multi-version support #19016
• Relates backport: post a backport checklist automatically on pull requests targeting main #19213
• Blocks backport: deploy backport-branch automation workflows to active backport branches #19262

---

This PR was generated with the assistance of Claude (claude-opus-5).

[infra-vault-gh-plugin-prod]

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History

• 💚 Build #47441 succeeded 0dbbd67

cc @mrodm