Create public Semgrep workflow definition

Due to some kind of undocumented Github problem, a public repo can only be required to run workflows from other public repos.

.github/workflows/semgrep.yml

@@ -0,0 +1,31 @@
+name: Semgrep
+on:
+  workflow_dispatch: {}
+  pull_request:
+    types: [opened, reopened, synchronize]
+  merge_group:
+  push:
+    branches:
+      - main
+      - master
+    paths:
+      - .github/workflows/semgrep.yml
+jobs:
+  semgrep:
+    name: semgrep/ci
+    runs-on: ubuntu-latest
+    permissions:
+      packages: read
+      contents: read
+    env:
+      SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
+    container:
+      image: ghcr.io/temporalio/semgrep:latest
+      credentials:
+        username: ${{ github.actor }}
+        password: ${{ secrets.github_token }}
+    if: (github.actor != 'dependabot[bot]')
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run semgrep ci
+        run: semgrep ci