Add scripts/stage-plugins.sh and run-sample.sh for running the engine locally

Contracts and Implementation build into two separate bin/ folders, so
there's nowhere valid to point --plugins at without a staging step first
— every manual verification this session has done that staging by hand.
stage-plugins.sh does it for every plugin in one pass; run-sample.sh
stages then runs samples/WindowDemo windowed, forwarding extra args to
`engine run` (e.g. --screenshot).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N1qPfzq8TDCUMFMV3UwV5N

.gitignore

@@ -18,6 +18,9 @@ Thumbs.db
 out/
 artifacts/
 
+## scripts/stage-plugins.sh's output — regenerated from plugins/, not source.
+.stage/
+
 ## ImGui's own persisted window layout (position/size/open-state) — a
 ## per-machine editing session preference, not project source, same
 ## category as an IDE's own workspace state.