Ignore the local prototype_reference design HTMLs

Two hand-written HTML mockups sitting in the project root, not source. Adding
them to the ignore list alongside the already-removed reference_code entry so
they cannot be swept into a commit by a broad `git add`.

.gitignore

@@ -20,8 +20,9 @@ packages/server/dist
 ROADMAP.md
 pnpm-lock.yaml
 pnpm-workspace.yaml
-# Reference snapshot (read-only, not part of this project)
+# Local design prototypes and reference snapshots (read-only, not part of this project)
 reference_code/
+prototype_reference/
 
 # Server data
 packages/server/data/