ci: move the PR template check into its own workflow so an edit re-checks it

[ion-alpha-dev]

What

Moves the PR body follows template job out of ci.yml into a new .github/workflows/pr-body.yml, which triggers on edited as well as the usual events. The check itself is unchanged, including its name and the bot waiver.

Why

The check could only be satisfied by pushing a commit, which is absurd for a check about the pull request body.

Editing a body fired nothing: ci.yml triggers on [opened, synchronize, reopened, ready_for_review] and not edited, so a failing check just sat at its old result. Re-running the workflow did not help either, because the job reads the body from github.event.pull_request.body and a re-run replays the original event payload. It re-read the body as it was when the PR opened and failed identically, which reads as "my edit did not save". Hit live on #423 and #424: both bodies were corrected and both checks stayed red.

Adding edited to ci.yml would have been worse. Its concurrency group is keyed on the ref with cancel-in-progress, so editing a title would cancel an in-flight matrix run, and ci-success fails on a cancelled job. A title tweak could turn a green PR red.

A separate workflow gets the edited trigger with its own concurrency group, so a body edit costs one short job instead of the full matrix, and nothing gets cancelled.

How to verify

• Open a PR with a body missing ## What, confirm PR body follows template fails.
• Edit the body to add the sections. The check should re-run on its own and go green, with no push and no other job re-running.
• Edit a title while the matrix is running and confirm the matrix is not cancelled.
• On this PR: only PR body follows template should come from the new workflow, and CI success should still gate everything else.

Notes for reviewers

• The check name is deliberately unchanged (PR body follows template), so the main protection ruleset and any saved status context see the same name. The ruleset requires only CI success, so there is no settings change needed to merge this.
• Follow-up you may want: ci-success no longer has pr-body in its needs, so the template check is no longer transitively required. If it should stay blocking, add PR body follows template to the ruleset's required checks. Adding it is safe at any time, before or after this merges, because the name does not change.
• Two things left deliberately alone, flag if you disagree: the bot waiver (AUTHOR_TYPE = Bot skips the check), and reading the body from the event payload rather than the API. The payload read was a security decision and the edited trigger removes the reason to change it.
• Task: d184a8b0-ff35-4b67-86ba-4f4cde682b6a

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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