docs: simplify repository truth architecture (#295)

Modularize durable truth, bound always-loaded instructions, and keep domain-document size checks advisory. Local cross-platform repository hygiene checks pass; GitHub-hosted steps were not started because of the account billing state.

.github/workflows/repository-hygiene.yml

@@ -1,6 +1,7 @@
 name: CI
 
 on:
+  pull_request:
   push:
     branches:
       - main
@@ -20,7 +21,7 @@ jobs:
       - name: Validate tracked project state
         run: sh scripts/check-project-state.sh
         env:
-          SCREENER_BASE_SHA: ${{ github.event.before }}
+          SCREENER_BASE_SHA: ${{ github.event.pull_request.base.sha || github.event.before }}
       - name: Set up Go
         uses: actions/setup-go@v6
         with: