chore: drop tracked output/ sample in favor of CI-built demo gallery (#4)

The dracula sample under output/dracula/ was a reference render for the
README. Now that demo/ auto-generates every card × every theme on each
push, the sample is redundant — and keeping it trackers means every
behavioral tweak shows up as a diff in committed SVGs.

- Delete output/dracula/*.svg.
- .gitignore: collapse `output/*` + `!output/dracula/` to a plain `output/`.
- demo.yml: drop `output/**` from paths-ignore (no longer part of any push).
- README / docs: point readers at demo/ instead of output/dracula/.

.gitignore

@@ -27,10 +27,9 @@ go.work.sum
 # env file
 .env
 
-# Generated SVG cards from local runs — keep the dracula sample tracked so the
-# README's embedded previews resolve; other themes are rebuilt on demand.
-output/*
-!output/dracula/
+# Generated SVG cards from local runs. Use the demo/ gallery (auto-generated
+# in CI) as the reference render instead of committing a sample here.
+output/
 
 # Editor/IDE
 # .idea/