CI: Re-run lint when a PR's base branch changes

[bjorkert]

What

Add edited to the lint workflow's pull_request trigger, guarded so the SwiftFormat job only re-runs when the PR's base branch actually changed (not on routine title/description edits).

on:
  pull_request:
    types: [opened, synchronize, reopened, edited]

jobs:
  swiftformat:
    if: ${{ github.event.action != 'edited' || github.event.changes.base != null }}

Why

A real PR got stuck "pending" indefinitely (#653) and this is the fix.

The default pull_request trigger types are [opened, synchronize, reopened]. Changing a PR's base branch fires pull_request: edited — which none of those cover — so the lint workflow does not re-run on a retarget.

That matters because SwiftFormat is a required status check on dev. The failure sequence:

1. A PR is opened against main (common for new/outside contributors).
2. It gets retargeted main → dev. This fires only edited, so lint does not re-run.
3. The dev ruleset now requires SwiftFormat, but no run ever reported it on the new base. GitHub shows it as "Expected — waiting for status to be reported" and the PR stays BLOCKED forever, with no run to even approve.

Re-running lint on a base change ensures the required check actually reports against the new base. The if guard keeps it from re-linting on every title/body edit (which also fire edited).

Note: this does not change the fork-PR approval gate — runs on outside-contributor forks still need a maintainer to approve them — but it guarantees there is a run to approve after a retarget, instead of a phantom required check.

[marionbarker]

approve from code review