Review workflows that only run on pull requests against main

[caugner]

Summary

Several workflows across the MDN org restrict pull_request triggers to branches: [main], e.g. PR Test in mdn/translated-content:

on:
  pull_request:
    branches:
      - main

This prevents the workflow from running on stacked PRs (PRs that target another PR's branch instead of main). Since these workflows typically compute their diff from the PR's own base and head SHAs, the filter seems unnecessary and may be unintentional.

Example

mdn/translated-content#38394 targets another branch, so PR Test never ran, and hence PR review companion never deployed a preview. Lint content (no branch filter) ran as expected.

Proposal

Review the pull_request triggers across MDN repos (mdn/content, mdn/translated-content, mdn/fred, mdn/rari, mdn/yari, mdn/workflows, ...) and drop the branches: [main] filter where the workflow does not actually depend on the base branch.

[caugner]

Scanned all workflow files in non-archived mdn repos. These pull_request workflows have a branches: [main] filter and therefore skip stacked PRs:

Repo	Workflow	Name
mdn/translated-content	pr-test.yml	PR Test
mdn/translated-content	markdown-lint.yml	Markdownlint (global)
mdn/translated-content	pr-check_json.yml	JSON lint
mdn/translated-content	pr-check_redirects.yml	Check Redirects
mdn/translated-content	pr-check_yml.yml	Lint YAML
mdn/translated-content-de	pr-test.yml	PR Test
mdn/translated-content-de	markdown-lint.yml	Markdownlint (All files)
mdn/translated-content-de	pr-check_json.yml	JSON lint
mdn/translated-content-de	pr-check_redirects.yml	Check Redirects
mdn/translated-content-de	pr-check_yml.yml	Lint YAML
mdn/blog	build.yml	Build
mdn/blog	static_checks.yml	Tests
mdn/differy	rust.yml	Rust

For comparison, mdn/content's pr-test.yml has no branch filter.

Several pull_request_target workflows (PR Needs Rebase, System file changes, auto-merge, etc.) also filter on main, but that is likely intentional since they act on the PR from the base branch context.