CI: Use pull_request_target for PR approval check (#4615)

The PR approval check workflow uses `pull_request`, which does not grant
the `GITHUB_TOKEN` write permissions for commit statuses on fork PRs.
This causes the check to silently fail on external contributions.

Switches to `pull_request_target`, which runs in the context of the base
branch and has the necessary permissions.

**Security notes** (also documented as comments in the workflow file):
- `pull_request_target` grants write access to the repository. This is
safe here because the workflow **only reads PR metadata via the GitHub
API** and never checks out, builds, or executes code from the PR branch.
- A clear `SECURITY` comment block at the top of the file explains why
`pull_request_target` is used and warns against adding a checkout step.
- An additional inline comment on the job warns against adding checkout
steps.

Co-authored-by: clockwork-labs-bot <clockwork-labs-bot@users.noreply.github.com>

.github/workflows/pr_approval_check.yml

@@ -1,7 +1,17 @@
 name: Review Checks
 
+# SECURITY: This workflow uses pull_request_target so that it has write access to
+# set commit statuses on external (fork) PRs. pull_request_target runs in the
+# context of the base branch, which grants the GITHUB_TOKEN write permissions
+# that a regular pull_request event on a fork would not have.
+#
+# IMPORTANT: This workflow must NEVER check out, build, or execute code from the
+# PR branch. Doing so would allow a malicious fork to run arbitrary code with
+# write access to the repository. This workflow only reads PR metadata via the
+# GitHub API, which is safe.
+
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, synchronize, reopened]
   pull_request_review:
     types: [submitted, dismissed]
@@ -21,6 +31,7 @@ jobs:
     name: Set approval status
     runs-on: ubuntu-latest
 
+    # SECURITY: Do not add a checkout step to this job. See comment at the top of this file.
     steps:
       - name: Evaluate and publish approval status
         uses: actions/github-script@v7