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/publish-immutable-actions.yml

@@ -5,6 +5,8 @@ on:
     types: [released]
   workflow_dispatch:
 
+permissions: {}
+
 jobs:
   publish:
     runs-on: ubuntu-latest
@@ -16,6 +18,8 @@ jobs:
     steps:
       - name: Checking out
         uses: actions/checkout@v7
+        with:
+          persist-credentials: false
       - name: Publish
         id: publish
         uses: actions/publish-immutable-action@v0.0.4