Remove the proposal-loop demo page

It existed to prove the round-trip (proposed -> approved -> PR ->
lint -> merge -> live) and it did. Its job is done.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

questions/test-proposal-demo.yaml

@@ -1,15 +0,0 @@
-id: /test-proposal-demo
-route: ^/test-proposal-demo
-name: Proposal loop verified
-description: >
-  This page was proposed through /develop-proposal, approved on
-  /develop, committed by the development-commit automation, and merged
-  only after lint passed. Safe to delete.
-alternatives:
-  - name: How it got here
-    description: ""
-    features:
-      - name: Dialogue-driven development
-        description: >
-          The same question architecture that runs this site also
-          changes it.