chore: adopt the shared PR review bot

.github/workflows/assign-pr-reviewer.yml

@@ -0,0 +1,44 @@
+name: Assign PR Reviewer
+
+# Assigns a reviewer and assignee to open PRs that have neither, and DMs them on
+# Slack. Logic lives in cm-agents so every repo runs the same code rather than a
+# copy that drifts: causematch/cm-agents/actions/pr-review-bot.
+#
+# Roster and Slack token are organisation-level, so nothing needs configuring here.
+
+on:
+  pull_request:
+    types: [opened, reopened, ready_for_review]
+  schedule:
+    # Safety net for anything the pull_request trigger missed. GitHub
+    # deprioritises schedules on busy repos, so in practice this fires closer to
+    # hourly than every 15 minutes.
+    - cron: '*/15 * * * *'
+  workflow_dispatch:
+
+concurrency:
+  # One shared group, not per-PR: a pull_request run and a scheduled sweep must
+  # serialise against each other, or both can read "needs a reviewer" before
+  # either has written anything and assign two different people.
+  group: assign-pr-reviewer
+  cancel-in-progress: false
+
+jobs:
+  assign:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 10
+    permissions:
+      # All three are load-bearing and an explicit block makes every unlisted
+      # scope `none`: without contents:read the action itself cannot be fetched,
+      # without issues:write the assignee step fails (assignees live on the
+      # issues endpoint even for a PR). No test catches a missing scope.
+      contents: read
+      issues: write
+      pull-requests: write
+    steps:
+      - uses: causematch/cm-agents/actions/pr-review-bot@master
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          roster: ${{ vars.PR_REVIEWER_ROSTER }}
+          slack-token: ${{ secrets.SLACK_BOT_TOKEN }}
+          weekly-target: ${{ vars.PR_REVIEWER_WEEKLY_TARGET }}