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/sdlc/SKILL.md

@@ -73,7 +73,7 @@ Each promotion is a PR reviewed and merged by its gate owner:
 
 ### Pipeline A: Plugin/Feature Changes
 
-```
+```text
 Engineer → PR to dev → self-merge → deploys to dev
 → Engineer validates on dev
 → PR from dev → uat → QA reviews → QA merges
@@ -88,7 +88,7 @@ Applies to changes in `headlamp-*-plugin/` repos (plugin code, features, bug fix
 
 ### Pipeline B: Infrastructure Changes (No UI Impact)
 
-```
+```text
 Engineer → PR to main → CI passes → QA reviews → QA merges
 → Production
 ```

skills/sdlc/sdlc-diagram.md

@@ -132,4 +132,3 @@ flowchart LR
     class UATB uat
     class MAIN prod
 ```
-