ci(test): skip test-trigger-other-job on fork PRs

The test-trigger-other-job job runs `gh workflow run`, which requires
`actions: write` on the GITHUB_TOKEN. PRs from forks receive a
read-only token by default, so this job has been failing on every
fork-originated PR with:

    could not create workflow dispatch event: HTTP 403:
    Resource not accessible by integration

Examples include #15 (dependabot) and any contributor PR.

Skip this job when the PR head is in a different repository than the
base. The job continues to run on PRs from same-repo branches and on
direct pushes, where the token has the required permissions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bearomorphism

.github/workflows/test.yaml

@@ -81,6 +81,10 @@ jobs:
             assert.ok(changelogContent.includes('### Fix'), 'Expected changelog to contain a header for fixes');
   test-trigger-other-job:
     runs-on: ubuntu-latest
+    # Fork PRs receive a read-only GITHUB_TOKEN, so `gh workflow run`
+    # below would fail with HTTP 403 ("Resource not accessible by
+    # integration"). Skip this job for PRs from forks.
+    if: github.event.pull_request.head.repo.full_name == github.repository
     steps:
       - uses: actions/checkout@v6
         with: