fix: resolve pre-existing markdownlint errors blocking CI

- sdlc-diagram.md: remove double blank line (MD012)
- sdlc/SKILL.md: add 'text' lang to fenced code blocks (MD040, 2 instances)
- uat/SKILL.md: add trailing newline (MD047)

These pre-existing issues were present on main and caused CI to fail
on any incoming PR.

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

skills/uat/SKILL.md

@@ -66,4 +66,4 @@ If `UAT_PLAYBOOK.md` does not exist for a plugin, treat that as a gap — report
 
 - **Approve** the `uat→main` promotion when all applicable test steps from the playbook pass and no console errors are present
 - **Request changes** when any test step fails — include specific failing steps, observed results vs. expected results, and failure screenshots
-- **Block** if the plugin fails to load entirely — escalate to CTO as a deployment issue requiring immediate resolution
+- **Block** if the plugin fails to load entirely — escalate to CTO as a deployment issue requiring immediate resolution