ci: migrate from Drone to GitHub Actions (self-hosted runners)

.github/CODEOWNERS

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+# Changes under .github/ require code-owner review: workflow files decide what
+# runs in CI and with which credentials.
+/.github/ @livingdocsIO/devops @livingdocsIO/admins

.github/workflows/ci.yml

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+# Replaces .drone.yml.
+#
+# GitHub-hosted runners on purpose: this repo is public, and self-hosted
+# runners must never be reachable from public repositories (a fork PR can run
+# arbitrary code). GitHub-hosted minutes are free for public repos.
+name: CI
+
+on:
+  push:
+    branches: ['**']
+    tags: ['**']
+  pull_request:
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Lint, build and test
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    # Chrome for the karma suite comes from the image, not the runner host.
+    container:
+      image: cypress/browsers:node-22.21.0-chrome-141.0.7390.107-1-ff-144.0-edge-141.0.3537.92-1
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Lint
+        run: npm run lint
+
+      - name: Build
+        run: npm run build -s
+
+      # karma.conf.js already runs ChromeHeadlessNoSandbox, single-run
+      - name: Test
+        run: npm run test:ci -s
+
+  release:
+    name: Semantic release
+    # Only publishes once the whole suite is green on the default branch.
+    if: github.ref == 'refs/heads/master' && github.event_name == 'push'
+    needs: test
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    # npm rejects concurrent publishes of the same package, so a repo-wide lock
+    concurrency:
+      group: release
+      cancel-in-progress: false
+    permissions:
+      contents: read # the App token below does the writing
+    steps:
+      # Tags pushed with the built-in GITHUB_TOKEN never trigger workflows, so
+      # semantic-release pushes with a livingdocs-release App token instead —
+      # that is what makes the tag fire the gh-pages job below.
+      - uses: actions/create-github-app-token@v2
+        id: app-token
+        with:
+          app-id: ${{ vars.RELEASE_APP_ID }}
+          private-key: ${{ secrets.RELEASE_APP_PRIVATE_KEY }}
+          permission-contents: write
+          permission-issues: write
+          permission-pull-requests: write
+
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # semantic-release needs full history and tags
+          token: ${{ steps.app-token.outputs.token }}
+          # CRITICAL: without this the persisted default GITHUB_TOKEN wins the
+          # push, the tag is attributed to github-actions[bot], and the
+          # tag-triggered gh-pages job never fires.
+          persist-credentials: false
+
+      - name: Release
+        uses: docker://livingdocs/semantic-release:v3.0.3
+        env:
+          GH_TOKEN: ${{ steps.app-token.outputs.token }}
+          NPM_TOKEN: ${{ secrets.NPM_TOKEN_PUBLISH }}
+
+  publish-docs:
+    name: Publish examples to gh-pages
+    if: startsWith(github.ref, 'refs/tags/')
+    needs: test
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    container:
+      image: cypress/browsers:node-22.21.0-chrome-141.0.7390.107-1-ff-144.0-edge-141.0.3537.92-1
+    permissions:
+      contents: write # pushes the built docs to the gh-pages branch
+    steps:
+      - uses: actions/checkout@v4
+
+      - run: npm ci
+
+      - name: Build docs
+        run: npm run build -s
+
+      - name: Publish to gh-pages
+        uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e # v4.0.0
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./examples
+          keep_files: false