fix: restore CI workflow with markdownlint config

- Restore .github/workflows/ci.yaml that was deleted in April cleanup
- Add .markdownlint.yaml with relaxed rules for skill files
- Fix MD040 error in skills/sdlc/SKILL.md (add language to code block)
- Allows line lengths > 80, emphasis-as-headings, compact tables

Fixes CI failures on 'Merge POLICIES.md content into agent instruction bundles' commit.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.markdownlint.yaml

@@ -0,0 +1,12 @@
+# Markdownlint configuration for the org repo.
+# Skill files intentionally use longer lines and emphasis-as-headings.
+# Allow these patterns for skills directory.
+
+# Line length is disabled for skill documentation
+MD013: false
+
+# Emphasis used as headings is allowed in skill files
+MD036: false
+
+# Compact table style is allowed
+MD060: false