ci: check PR description for a Phabricator task Git trailer

.github/scripts/check-pr-git-trailers.sh

@@ -0,0 +1,71 @@
+#!/usr/bin/env bash
+# Requires: git, jq, and curl
+set -euo pipefail
+
+# jq's `// ""` substitutes an empty string if the `.pull_request.body` is null (or false)
+# https://jqlang.org/manual/#alternative-operator
+pr_description=$(jq -r '.pull_request.body // ""' "${GITHUB_EVENT_PATH}")
+# A PR description is not a patch so ignore any Markdown dividers (---)
+git_trailers=$(printf '%s\n' "${pr_description}" | git interpret-trailers --parse --no-divider)
+# -x requires a whole-line match
+if grep -x 'Bug: T[1-9][0-9]*' <<< "${git_trailers}" > /dev/null; then
+    echo 'Found a Git trailer for a likely Phabricator task.'
+    exit 0
+fi
+
+# Wikibase Cloud (Kanban Board)
+# https://phabricator.wikimedia.org/project/board/8048/
+# PHID found via:
+#   curl https://phabricator.wikimedia.org/api/project.search \
+#     -d api.token=<api-token> \
+#     -d constraints[ids][0]=8048
+project_PHID='PHID-PROJ-pmh2lan2qqkhxh43p66x'
+# Waiting for Peer Review column
+# https://phabricator.wikimedia.org/project/board/8048/column/35965/
+# PHID found via:
+#   curl https://phabricator.wikimedia.org/api/project.column.search \
+#     -d api.token=api-token \
+#     -d constraints[ids][0]=35965
+column_PHID='PHID-PCOL-3zqq4vuit77cj3l4pl7y'
+
+# URL encode directly from the event to preserve special characters and newlines
+task_url=$(jq -r --arg project_PHID "${project_PHID}" --arg column_PHID "${column_PHID}" '
+    .pull_request as $pr |
+    "https://phabricator.wikimedia.org/maniphest/task/edit/form/1" +
+    "?project=\($project_PHID)&column=\($column_PHID)" +
+    "&title=" + ($pr.title | @uri) +
+    "&description=" + ((($pr.body // "") + "\n\n---\n\n**PRs**:\n- [] \($pr.html_url)") | @uri)
+' "${GITHUB_EVENT_PATH}")
+
+author=$(jq -r '.pull_request.user.login' "${GITHUB_EVENT_PATH}")
+pr_comment=$(cat <<EOF
+@${author}, please add any related Phabricator tasks as git trailers at the end of the PR description, separated from the text by a blank line.
+
+For example (replace the example task IDs with your related tasks):
+
+\`\`\`text
+Example PR description text
+
+Bug: T12345
+Bug: T67890
+\`\`\`
+
+You can also create a new task with the same title and description as this PR using the link below, then add its task ID as a \`Bug:\` trailer:
+
+- [Create task in the "Waiting for Peer Review" column](${task_url})
+EOF
+)
+
+pr_number=$(jq -r '.pull_request.number' "${GITHUB_EVENT_PATH}")
+# --silent hides the progress meter; --show-error keeps diagnostics on stderr.
+# --fail-with-body returns a failure status for HTTP errors while writing the body to stdout.
+jq -n --arg body "${pr_comment}" '{body: $body}' |
+    curl --silent --show-error --fail-with-body --max-time 30 \
+        --request POST \
+        --header "Authorization: Bearer ${GH_TOKEN}" \
+        --header 'Accept: application/vnd.github+json' \
+        --header 'Content-Type: application/json' \
+        --data-binary @- \
+        "${GITHUB_API_URL}/repos/${GITHUB_REPOSITORY}/issues/${pr_number}/comments"
+
+echo 'Created reminder comment on the pull request.'

.github/workflows/pullrequest.check.yml

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+name: Check PR description for a Phabricator task Git trailer
+
+on:
+  pull_request:
+    types: [opened, reopened]
+
+jobs:
+  check-pr-git-trailers:
+    permissions:
+      pull-requests: write
+    if: github.event.pull_request.user.login != 'dependabot[bot]'
+
+    runs-on: ubuntu-24.04
+
+    steps:
+
+      - name: Checkout workflow scripts
+        uses: actions/checkout@v7
+        with:
+          sparse-checkout: .github/scripts
+
+      - name: Check trailers and request related tasks
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: bash .github/scripts/check-pr-git-trailers.sh