fix: allow fork PRs to pass validation when commenting fails (#15)

Add continue-on-error to the PR comment step. GITHUB_TOKEN is
read-only for fork PRs, so the comment POST returns HTTP 403.
Validation results are still reported via job annotations and
exit code — the comment is a nice-to-have, not a gate.

Fixes #13

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

Author: lewing

.github/workflows/pr-validation.yml

@@ -502,6 +502,7 @@ jobs:
 
       - name: Post or update PR comment
         if: always() && github.event_name == 'pull_request' && steps.validate.outputs.has_comment == 'true'
+        continue-on-error: true  # GITHUB_TOKEN is read-only for fork PRs (dotnet/arcade-skills#13)
         env:
           GH_TOKEN: ${{ github.token }}
         shell: bash