Add markdownlint and link-checking CI

.github/workflows/checks.yml

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+name: checks
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+  schedule:
+    # Weekly, so external link rot surfaces on its own rather than at the next edit.
+    - cron: "17 9 * * 1"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: checks-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  markdown:
+    name: markdownlint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: DavidAnson/markdownlint-cli2-action@21c1be1b93ad9ed58fa840aacc3f279cde2a72ff # v24.2.0
+        with:
+          globs: "**/*.md"
+
+      - name: Pin the MD041 exception
+        # profile/README.md carries a scoped MD041 disable, because the brand
+        # lockup has to precede the h1. That suppression is only honest while the
+        # h1 it points at actually exists — otherwise deleting the heading would
+        # silently reintroduce the accessibility defect with no check failing.
+        # Assert the property the rule was protecting instead of trusting it.
+        run: |
+          count=$(grep -c '^# ' profile/README.md || true)
+          if [ "$count" -ne 1 ]; then
+            echo "::error file=profile/README.md::expected exactly 1 top-level heading, found $count"
+            exit 1
+          fi
+          echo "profile/README.md has exactly one h1"
+
+  links:
+    name: link check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: lycheeverse/lychee-action@e7477775783ea5526144ba13e8db5eec57747ce8 # v2.9.0
+        with:
+          args: >-
+            --no-progress
+            --include-verbatim
+            --max-retries 3
+            --timeout 20
+            --accept 200,206,301,302,403
+            .
+          fail: true

.markdownlint-cli2.yaml

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+# The profile README is a rendered GitHub page, not a document tree, so a few
+# rules that assume prose-document structure are relaxed deliberately.
+config:
+  # Line length: prose is wrapped by hand at ~80. Tables, links, and the inline
+  # HTML the org profile needs cannot always honour that.
+  MD013:
+    line_length: 100
+    tables: false
+    code_blocks: false
+  # GitHub renders raw HTML in profile READMEs and it is the only way to centre
+  # the brand lockup. Allowed, but kept to the block elements actually needed.
+  MD033:
+    allowed_elements: [div, img, sub, b, a, br, picture, source]
+  # Duplicate headings are fine across separate files in this repo.
+  MD024:
+    siblings_only: true
+
+globs:
+  - "**/*.md"
+  - "!LICENSE"