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/.

docs/codebase-summary.md

@@ -45,8 +45,8 @@ ghstats/
 │   └── demo.yml                         # Renders every theme for the repo owner on push to main
 ├── docs/                                # This directory
 ├── plans/                               # Research reports + implementation plans
-├── demo/                                # Auto-generated gallery — every card × every theme + README
-└── output/dracula/                      # Sample committed; other themes gitignored
+└── demo/                                # Auto-generated gallery — every card × every theme + README
+                                         # (`output/` is entirely gitignored; see demo/ for reference renders)
 ```
 
 ## Module responsibilities