chore: Harden workflows: least-privilege permissions + zizmor integration (#1039)

* Harden workflows with least-privilege permissions and zizmor

Apply GitHub Actions security best practices to the action's own
workflows and integrate zizmor to catch regressions.

- Add explicit least-privilege `permissions:` to every workflow
  (contents: read for read-only workflows; default-deny `{}` with
  job-scoped grants for codeql, publish-immutable-actions and
  update-config-files).
- Set `persist-credentials: false` on all checkout steps that don't
  need the GITHUB_TOKEN afterwards.
- Move `${{ ... }}` expansions out of `run:` blocks into `env:` vars
  to avoid template injection.
- Pin the alpine container image (alpine:latest -> alpine:3.21).
- Add a zizmor CI workflow that uploads SARIF to code scanning, plus a
  `.github/zizmor.yml` pinning policy (ref-pin for actions/* and
  github/*, hash-pin for third-party actions).

zizmor now reports no findings (offline and online).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Fix indentation of if: in zizmor SARIF upload step

The `if:` key on the "Upload SARIF results to code scanning" step had no
indentation, producing invalid YAML ("Nested mappings are not allowed in
compact mappings"). This broke `npm run format-check` (prettier) in Basic
validation.

Indent `if:` to 8 spaces so it nests under the step alongside uses/with.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

.github/workflows/zizmor.yml

@@ -0,0 +1,48 @@
+name: Security analysis with zizmor
+
+on:
+  push:
+    branches:
+      - main
+      - releases/*
+    paths-ignore:
+      - '**.md'
+  pull_request:
+    paths-ignore:
+      - '**.md'
+  workflow_dispatch:
+
+permissions: {}
+
+jobs:
+  zizmor:
+    name: Analyze workflows with zizmor
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      security-events: write # to upload SARIF results to code scanning
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+        with:
+          persist-credentials: false
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.x'
+
+      - name: Install zizmor
+        run: pip install zizmor
+
+      - name: Run zizmor
+        run: zizmor --format sarif .github/workflows/ > zizmor.sarif
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Upload SARIF results to code scanning
+        if: always() && (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository)
+        uses: github/codeql-action/upload-sarif@v3
+        with:
+          sarif_file: zizmor.sarif
+          category: zizmor