Add issue triage comments workflow (#4450)

## Description

Adds a workflow that posts triage comments based on issue labels. Labels
are now applied by @coderabbitai, which only replies when @-mentioned
and so cannot post them itself.

Removes `needs-repro.yml`, `needs-more-info.yml` and `platforms.yml`,
which previously did both the labeling and the commenting. They are
already disabled in the repository settings. `close-when-stale.yml` is
unchanged.

## Test plan

Workflows only run from the default branch, so after merge: open an
issue with no reproduction and confirm the comment appears, then remove
the label and confirm it is deleted.