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it works" is rewritten as the native-surface map; the superseded F6/F7/F9/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

@@ -10,7 +10,7 @@ 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?.trackChangesPreviewController, "exports preview controller");
+  assert.ok(api?.editFlow && api?.proposalController, "exports edit flow + proposal controller");
   return api;
 }
 
@@ -37,8 +37,6 @@ suite("#60 live turn progress (additive + cancel)", () => {
     const doc = await freshDoc("docs/live60-additive.md", "# Title\n\nOld paragraph.\n");
     const api = await getApi();
     const editFlow = api.editFlow;
-    await vscode.commands.executeCommand("cowriting.showTrackChangesPreview");
-    await settle();
 
     // The stub honors opts.onProgress (emitting synthetic snapshots) but returns
     // the same rewrite — proposals must be unaffected by progress events.
@@ -63,8 +61,6 @@ suite("#60 live turn progress (additive + cancel)", () => {
     const doc = await freshDoc("docs/live60-cancel.md", "# Title\n\nOld paragraph.\n");
     const api = await getApi();
     const editFlow = api.editFlow;
-    await vscode.commands.executeCommand("cowriting.showTrackChangesPreview");
-    await settle();
 
     // The stub throws ONLY when the aborted signal reached it — so if opts.signal
     // failed to thread through runEditAndPropose, the stub would instead return a