feat!: sunset the review-panel webview — built-in preview + native diff are the review surfaces (D3/D17, spec §6.10); Keep/Reject lens copy (PUC-2)

Deletes the last bespoke UI surface (TrackChangesPreviewController + its
sealed webview client assets); every entry point re-points at native VS
Code chrome per spec §5: the #41 right-click entries become
cowriting.openReviewPreview (enter coediting if needed -> "Open Preview to
the Side"), Ctrl+Alt+R/Cmd+Alt+R moves to cowriting.reviewChanges (native
diff), and the F12 CodeLens per-proposal titles read "Keep"/"Reject" with
a top-of-file "Keep all (N)"/"Reject all" pair once >=2 proposals are
pending. EditFlow drops its own askClaude/askEditInstruction (the
webview's only caller) and its now-unused constructor params.

Coverage that lived only in the webview's test seams (renderHtmlFor,
receiveMessage, isOpen, ...) moves to direct calls against the surviving
controllers/pure renderReview (test/e2e/suite/helpers.ts gains a shared
renderHtmlFor probe); a genuine gap (pending-proposal + unchanged-block
rendering) is backfilled in test/previewAnnotations.test.ts. README's "how
it works" is rewritten as the native-surface map; the superseded F6/F7/F9/
F10/F11 sections are kept as a marked historical record rather than
deleted outright.

292 unit + 91 E2E green.

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

test/e2e/suite/helpers.ts

@@ -6,6 +6,7 @@
 import * as assert from "node:assert";
 import * as vscode from "vscode";
 import type { CowritingApi } from "../../../src/extension";
+import { renderReview } from "../../../src/trackChangesModel";
 
 /** Activate the extension and return its exported API (asserts it is real). */
 export async function activateApi(): Promise<CowritingApi> {
@@ -15,6 +16,25 @@ export async function activateApi(): Promise<CowritingApi> {
   return api;
 }
 
+/**
+ * Task 8: the pure render probe the deleted TrackChangesPreviewController used
+ * to wrap (`renderHtmlFor`) — the sunset webview was a thin shell around this
+ * exact call (F6 baseline + F3 spans + F4 pending proposals -> `renderReview`).
+ * Suites that used to read `controller.renderHtmlFor(key)` now call this
+ * directly against the surviving controllers; no webview involved.
+ */
+export function renderHtmlFor(api: CowritingApi, doc: vscode.TextDocument, key: string): string {
+  const baseline = api.diffViewController.getBaseline(key);
+  const current = doc.getText();
+  return renderReview(
+    baseline?.text ?? current,
+    current,
+    api.attributionController.spansFor(doc),
+    api.proposalController.listProposals(doc),
+    { pinned: baseline?.reason === "pinned" },
+  );
+}
+
 /** A short fixed settle, for state that updates synchronously-ish after a command. */
 export function settle(): Promise<void> {
   return new Promise((r) => setTimeout(r, 150));