Lint this repo's Markdown in CI

.github/workflows/lint.yaml

@@ -0,0 +1,23 @@
+name: Lint
+
+# ci.yml in this repo is a reusable workflow for the library repos and never
+# runs here, so this repo's own Markdown needs a workflow of its own.
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+      - name: Setup Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.13'
+      - name: Install pre-commit
+        run: pip install pre-commit
+      - name: Run linters
+        run: pre-commit run --all-files

.pre-commit-config.yaml

@@ -0,0 +1,18 @@
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: "3e8a8703264a2f4a69428a0aa4dcb512790b2c8c"  # frozen: v6.0.0
+    hooks:
+      - id: check-json
+      - id: check-yaml
+      - id: check-merge-conflict
+      - id: end-of-file-fixer
+      - id: trailing-whitespace
+  - repo: https://github.com/igorshubovych/markdownlint-cli
+    rev: "5b5dddc4fb0f83c3ea1fc5616fa63e115dce83e0"  # frozen: v0.49.1
+    hooks:
+      - id: markdownlint
+        args:
+          - --fix
+        # GitHub injects this into the PR description, so it's a fragment
+        # rather than a document and deliberately has no top-level heading.
+        exclude: ^PULL_REQUEST_TEMPLATE/