ci: implement supply chain security and workflow linting

.github/workflows/supply-chain.yml

@@ -0,0 +1,64 @@
+name: Supply chain
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+  merge_group:
+  schedule:
+    - cron: "41 2 * * 1"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  cargo-deny:
+    name: cargo-deny ${{ matrix.checks }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          # Advisories can appear after code was merged. Keep this visible,
+          # but don't let a surprise upstream advisory nuke every unrelated PR.
+          - checks: advisories
+            continue-on-error: true
+
+          # These are policy choices in this repo. They should block PRs.
+          - checks: bans licenses sources
+            continue-on-error: false
+
+    continue-on-error: ${{ matrix.continue-on-error }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+        with:
+          persist-credentials: false
+
+      - name: Run cargo-deny
+        uses: EmbarkStudios/cargo-deny-action@v2
+        with:
+          command: check
+          arguments: --all-features
+          command-arguments: ${{ matrix.checks }}
+
+  actionlint:
+    name: actionlint
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+        with:
+          persist-credentials: false
+
+      - name: Run actionlint
+        uses: raven-actions/actionlint@v2