ci: add merge-blocking markdown format check

.github/workflows/markdown.yml

@@ -0,0 +1,22 @@
+name: Markdown
+
+# Runs unconditionally on every PR to main -- deliberately NO `paths:` filter.
+# This job is a required status check, and a path-filtered required check never
+# reports on PRs that touch no markdown, blocking them forever at
+# "Expected - waiting for status to be reported".
+
+on:
+  pull_request:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  # Job ID is the check-run context name the branch ruleset matches on.
+  markdown:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
+        with:
+          node-version: "24"
+      - run: npx --yes prettier@3.9.6 --check "**/*.md"

.prettierrc.json

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+{
+  "overrides": [
+    {
+      "files": ["**/*.md"],
+      "options": {
+        "proseWrap": "always",
+        "printWidth": 80,
+        "embeddedLanguageFormatting": "off"
+      }
+    }
+  ]
+}