fix: quote test-build verify step to fix YAML parse error

The "Verify output exists" step used an inline `run:` scalar containing
`successful: index.html` — the colon-space made YAML parse it as a nested
mapping, so GitHub rejected the whole workflow at load time (0s failure,
"workflow file issue"). Use a block scalar, as develop's workflow does.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: stiffneckjim

.github/workflows/test-build.yml

@@ -43,4 +43,5 @@ jobs:
         run: quarto render slides.qmd
 
       - name: Verify output exists
-        run: test -f index.html && echo "Build successful: index.html exists"
+        run: |
+          test -f index.html && echo "Build successful: index.html exists"