ci: harden supply chain (SHA pin actions, least-privilege permissions) (#475)

Pin three third-party actions to full commit SHAs with version comments
so dependabot can update them in place without exposing the pipeline to
upstream tag tampering:

  - aquasecurity/trivy-action@v0.35.0
  - peter-evans/create-pull-request@v8.1.1
  - googleapis/release-please-action@v4.4.0

Add a workflow-level permissions: contents: read default to ci.yml, with
update-screenshots keeping its explicit contents: write + pull-requests:
write override. Add the same default to docker-publish.yml (single job).

Expand .dockerignore to explicitly exclude .env*, *.key, *.pem, .github/,
docs/, website/, e2e/, tests/, test-results/, and playwright-report/ as
defense in depth against accidental build-context leaks. None of these
paths are read by any COPY step in Dockerfile.

.github/workflows/ci.yml

@@ -17,6 +17,11 @@ concurrency:
   group: ci-${{ github.ref }}
   cancel-in-progress: true
 
+# Least-privilege default for every job. Individual jobs may override (e.g.
+# update-screenshots needs contents: write + pull-requests: write to open a PR).
+permissions:
+  contents: read
+
 # ---------------------------------------------------------------------------
 # Build & Validation (PRs only, skipped for release-please PRs)
 # ---------------------------------------------------------------------------
@@ -118,7 +123,7 @@ jobs:
           cache-to: type=gha,mode=max
 
       - name: Scan image for vulnerabilities (Trivy)
-        uses: aquasecurity/trivy-action@master
+        uses: aquasecurity/trivy-action@57a97c7e7821a5776cebc9bb87c984fa69cba8f1 # v0.35.0
         with:
           image-ref: sencho:pr-test
           exit-code: '1'
@@ -195,7 +200,7 @@ jobs:
 
       - name: Open / update screenshots PR
         id: create-pr
-        uses: peter-evans/create-pull-request@v8
+        uses: peter-evans/create-pull-request@5f6978faf089d4d20b00c7766989d076bb2fc7f1 # v8.1.1
         with:
           token: ${{ secrets.DOCS_REPO_TOKEN }}
           branch: chore/refresh-screenshots

.github/workflows/docker-publish.yml

@@ -15,6 +15,8 @@ jobs:
     name: Push Docker image to Docker Hub
     runs-on: ubuntu-latest
     timeout-minutes: 30
+    permissions:
+      contents: read
     steps:
       - name: Check out the repo
         uses: actions/checkout@v6

.github/workflows/release-please.yml

@@ -13,7 +13,7 @@ jobs:
   release-please:
     runs-on: ubuntu-latest
     steps:
-      - uses: googleapis/release-please-action@v4
+      - uses: googleapis/release-please-action@16a9c90856f42705d54a6fda1823352bdc62cf38 # v4.4.0
         with:
           # GITHUB_TOKEN cannot trigger other workflows (GitHub security restriction).
           # Using DOCS_REPO_TOKEN (PAT) ensures the tag push from release-please