ci: add the release workflow

.github/workflows/release.yml

@@ -0,0 +1,59 @@
+name: Release
+
+on:
+  push:
+    branches: [main]
+
+# One release at a time. Two overlapping runs would race on the version bump.
+concurrency: release-${{ github.ref }}
+
+permissions:
+  contents: write # changesets commits version bumps and pushes tags
+  pull-requests: write # changesets opens and updates the "Version Packages" PR
+  id-token: write # OIDC — this is what replaces a long-lived npm token
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          # Changesets needs the history to work out what has already shipped.
+          fetch-depth: 0
+
+      - uses: pnpm/action-setup@v6
+
+      - uses: actions/setup-node@v7
+        with:
+          node-version-file: .nvmrc
+          cache: pnpm
+          registry-url: https://registry.npmjs.org
+
+      # Trusted publishing needs npm 11.5.1+; the runner's bundled npm is older.
+      - run: npm install -g npm@latest
+
+      - run: pnpm install --frozen-lockfile
+
+      # Build first: the workspace packages resolve through dist, so lint,
+      # typecheck and test all fail confusingly against an unbuilt tree.
+      - run: pnpm build
+      - run: pnpm lint
+      - run: pnpm typecheck
+      - run: pnpm test
+      - run: pnpm size
+
+      # Generated files are generated. If either has drifted the release would
+      # ship documentation that disagrees with the code it describes.
+      - run: pnpm docs:props && pnpm docs:agents
+      - name: Fail if generated docs are stale
+        run: git diff --exit-code || (echo "::error::Run pnpm docs:props and pnpm docs:agents, then commit" && exit 1)
+
+      - name: Create release PR or publish
+        uses: changesets/action@v1
+        with:
+          # While unreleased changesets exist this opens or updates the
+          # "Version Packages" PR. Merging that PR triggers the run that
+          # actually publishes, so a release is always a deliberate merge.
+          publish: pnpm changeset publish
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}