refactor(web-e2e): rename harness -> scaffold; add interaction coverage

The shared test module was named harness.ts inside a repo whose product
IS a harness — hopelessly ambiguous. Renamed to scaffold.ts with
launchWebScaffold/WebScaffold; tsconfig plane-split entries, the seam
JSDoc/README mentions, and both Agent Note languages updated.

Both scenarios gain a Playwright interaction step over the settled
transcript (after the golden capture, so committed aria surfaces stay
untouched): replay-round-trip clicks the reasoning fold open/closed over
wire-delivered state; seeded-history expands a read tool row rebuilt from
the cold log and asserts the recorded result text appears (read rows are
expand-in-place — rowExpands routes the click to the inline fold, not the
details column). test:web 30 passed | 1 skipped.

tsconfig.host.json

@@ -8,7 +8,7 @@
     "rewriteRelativeImportExtensions": false
   },
   "include": [
-    "apps/web/tests/harness.ts",
+    "apps/web/tests/scaffold.ts",
     "apps/web/tests/support.ts",
     "apps/web/tests/replay-round-trip.e2e.ts",
     "apps/web/tests/seeded-history.e2e.ts",