fix(ci): drop committed index.json so the freshness check stops fighting itself

The build stamps index.json with a fresh `generated_at` every run, so
the "is the file up to date" check could never pass. The pages
workflow already rebuilds the index before uploading the site
artifact, so the committed copy was redundant anyway. Now: index is
generated at deploy time only, CI just validates schema + smoke-tests
the build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: knhn1004

.github/workflows/ci.yml

@@ -18,12 +18,6 @@ jobs:
       - name: Validate rules against schema
         working-directory: tools
         run: bun run validate
-      - name: Build index and check it is up to date
+      - name: Build index (smoke test only)
         working-directory: tools
-        run: |
-          bun run build-index
-          if ! git diff --quiet -- ../site/data/index.json; then
-            echo "::error::site/data/index.json is stale. Run 'bun run build-index' in tools/ and commit the result."
-            git --no-pager diff -- ../site/data/index.json
-            exit 1
-          fi
+        run: bun run build-index

.gitignore

@@ -2,3 +2,7 @@ node_modules/
 .DS_Store
 *.log
 dist/
+
+# Generated at deploy time by the pages workflow. The Pages site fetches
+# data/index.json from the deployed artifact, not from the repo.
+site/data/index.json