fix: restore CI workflow with markdownlint config

The previous CI workflow was removed, leaving the agents repo with no CI.
This restores markdownlint-based CI with a config that disables rules
incompatible with agent prompt files (bare URLs, inline HTML, emphasis
headings). Uses ARC runners and actions/checkout@v6.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yaml

@@ -0,0 +1,17 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  lint:
+    runs-on: runners-privilegedescalation
+    steps:
+      - uses: actions/checkout@v6
+      - name: Lint Markdown
+        uses: DavidAnson/markdownlint-cli2-action@v19
+        with:
+          globs: "**/*.md"

.markdownlint.yaml

@@ -0,0 +1,6 @@
+# Markdownlint configuration for the agents repo.
+# Agent prompt files intentionally use bare URLs (curl commands),
+# inline HTML (template variables), and emphasis-as-headings.
+MD033: false  # no-inline-html
+MD034: false  # no-bare-urls
+MD036: false  # no-emphasis-as-heading