Use pull_request_target for PR notifications so fork PRs get Slack alerts

pull_request events from forks don't have access to repo secrets, so the
SLACK_WEBHOOK was empty and curl silently failed. pull_request_target runs
in the base repo context and has access to secrets. Also switched curl -s
to curl -sf so HTTP failures surface as workflow failures.

Author: jpr5

.github/workflows/notify-pr.yml

@@ -1,6 +1,6 @@
 name: PR Notification
 on:
-  pull_request:
+  pull_request_target:
     types: [opened]
 jobs:
   notify:
@@ -12,6 +12,6 @@ jobs:
           PR_URL="${{ github.event.pull_request.html_url }}"
           PR_AUTHOR="${{ github.event.pull_request.user.login }}"
           PR_NUM="${{ github.event.pull_request.number }}"
-          curl -s -X POST "${{ secrets.SLACK_WEBHOOK }}" \
+          curl -sf -X POST "${{ secrets.SLACK_WEBHOOK }}" \
             -H "Content-Type: application/json" \
             -d "{\"text\": \"🔀 New PR #${PR_NUM} on aimock by *${PR_AUTHOR}*: <${PR_URL}|${PR_TITLE}>\"}"