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it works" is rewritten as the native-surface map; the superseded F6/F7/F9/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/undoMarks.test.ts

@@ -4,6 +4,7 @@ import * as path from "path";
 import * as vscode from "vscode";
 import type { CowritingApi } from "../../../src/extension";
 import { undoWorks, UNDO_SKIP_REASON } from "./undoCapable";
+import { renderHtmlFor } from "./helpers";
 
 const WS = process.env.E2E_WORKSPACE!;
 const settle = () => new Promise((r) => setTimeout(r, 400));
@@ -11,7 +12,7 @@ const settle = () => new Promise((r) => setTimeout(r, 400));
 async function getApi(): Promise<CowritingApi> {
   const ext = vscode.extensions.getExtension("benstull.vscode-cowriting-plugin")!;
   const api = (await ext.activate()) as CowritingApi;
-  assert.ok(api?.attributionController && api?.trackChangesPreviewController, "exports attribution + preview");
+  assert.ok(api?.attributionController && api?.proposalController, "exports attribution + proposal");
   return api;
 }
 
@@ -83,7 +84,7 @@ suite("F10 #38 — undo does not mis-attribute restored text (host E2E, no LLM)"
 
     // And the on-state render must not color 'bravo' as human-authored.
     // Added blocks use cw-ins-human (not cw-by-human) since Task 3/44ef0a2.
-    const html = api.trackChangesPreviewController.renderHtmlFor(key);
+    const html = renderHtmlFor(api, doc, key);
     const bravoColoredHuman = /cw-ins-human[^<]*bravo|<[^>]+class="[^"]*cw-ins-human[^"]*"[^>]*>[^<]*bravo/.test(html);
     assert.ok(!bravoColoredHuman, "restored 'bravo' is not colored cw-ins-human in the preview");
   });