fix(workflows): label PRs from forks that follow the contributing template

On a fork PR, GITHUB_TOKEN is demoted to read-only regardless of the
workflow's permissions: block, so issues.addLabels() returns 403. The
label workflow silently works for PRs that skip the template (no
checkboxes ticked → no API call) and fails for PRs that actually
follow it — a hostile incentive against contributors who do the right
thing.

pull_request_target runs in the context of the base branch with full
declared permissions, which is the documented fix for this case. Safe
here because the workflow is metadata-only: it reads
context.payload.pull_request.body and calls addLabels. No checkout,
no PR-supplied code executes. A SECURITY comment is added above the
trigger to keep it that way.

Refs:
- https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request_target
- https://securitylab.github.com/resources/github-actions-preventing-pwn-requests/

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/label-pr.yml

@@ -1,7 +1,12 @@
 name: Label PR
 
+# SECURITY: this workflow runs with write access to the base repo on fork PRs,
+# because `pull_request_target` executes in the context of the base branch.
+# Keep it metadata-only — do NOT add actions/checkout or any step that
+# executes PR-supplied content (install scripts, build commands, etc.).
+# See https://securitylab.github.com/resources/github-actions-preventing-pwn-requests/
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, edited]
 
 jobs: