Add GitHub Actions workflow for deploy notifications (#23)

Posts a Slack message to the schedule channel whenever code is pushed
to main. Uses the existing Slack bot token stored as a GitHub secret.

.github/workflows/deploy-notify.yml

@@ -0,0 +1,35 @@
+name: Deploy Notification
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
+
+      - name: Extract version info from commit
+        id: info
+        run: |
+          MSG=$(git log -1 --pretty=%s)
+          echo "message=$MSG" >> "$GITHUB_OUTPUT"
+          # Count merge commits on main as version number
+          COUNT=$(git rev-list --count HEAD)
+          echo "count=$COUNT" >> "$GITHUB_OUTPUT"
+
+      - name: Post to Slack
+        env:
+          SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
+          SLACK_CHANNEL_ID: ${{ secrets.SLACK_CHANNEL_ID }}
+        run: |
+          curl -s -X POST https://slack.com/api/chat.postMessage \
+            -H "Authorization: Bearer $SLACK_BOT_TOKEN" \
+            -H "Content-Type: application/json" \
+            -d "$(jq -n \
+              --arg channel "$SLACK_CHANNEL_ID" \
+              --arg text ":rocket: *Deployed to production* — ${{ steps.info.outputs.message }}" \
+              '{channel: $channel, text: $text}')"