fix: correct markdownlint errors in existing skill files

- sdlc-diagram.md: remove extra trailing blank line (MD012)
- sdlc/SKILL.md lines 76,91: add language to fenced code blocks (MD040)
- uat/SKILL.md: verify trailing newline (MD047)

These pre-existing issues were surfaced by the expanded CI introduced
in the 'ci: move to .gitea/workflows and expand lint coverage' commit.

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
 ```
-