feat(f11): SLICE-5 — gateway, non-authorable disabling, docs (#43)

Closes the F11 feature: makes the toolbar surface reachable end to end and
guards the edit controls. Per spec §6.4/§6.5/§7.2 SLICE-5.

- package.json: cowriting.showTrackChangesPreview added to editor/title
  (when: editorLangId == markdown) — the minimal right-click → Open Review
  Preview gateway (#41/#42 expand it later).
- trackChangesPreview: the gateway command accepts the tab's resource Uri
  (palette/keybinding still fall back to the active editor); refresh() sends an
  `authorable` flag on both render messages; `editControlsEnabled` test seam.
- webview: disable Pin + Ask-Claude on a non-authorable doc (Annotations stays
  active — reading is always allowed); RenderMessage.authorable.
- host E2E: the editor/title gateway opens the preview + is markdown-guarded;
  edit controls disabled on a non-authorable (read-only-scheme) markdown doc.
- docs: docs/MANUAL-SMOKE-F11.md (live smoke, 10 steps) + README F11 section +
  intro line.

205 unit + 53 host E2E green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/e2e/suite/f11Toolbar.test.ts

@@ -155,4 +155,52 @@ suite("F11 preview toolbar (host E2E — message → seam wiring, no LLM)", () =
     assert.ok(entry, "editDocument has a commandPalette entry");
     assert.match(entry!.when ?? "", /editorLangId == markdown/, "guarded on markdown");
   });
+
+  // SLICE-5: the minimal right-click gateway lives in editor/title (markdown only).
+  test("the editor/title gateway opens the preview, and the menu entry is markdown-guarded (PUC-6)", async () => {
+    const pkg = JSON.parse(fs.readFileSync(path.join(__dirname, "../../../../package.json"), "utf8"));
+    const entry = (pkg.contributes.menus["editor/title"] as Array<{ command: string; when?: string }>).find(
+      (m) => m.command === "cowriting.showTrackChangesPreview",
+    );
+    assert.ok(entry, "showTrackChangesPreview is in editor/title");
+    assert.match(entry!.when ?? "", /editorLangId == markdown/, "gateway guarded on markdown");
+    // the command it invokes opens the panel.
+    const { key } = await freshDoc("docs/f11gw.md", "# F11 gateway\n\nReachable end to end.\n");
+    const api = await getApi();
+    await vscode.commands.executeCommand("cowriting.showTrackChangesPreview");
+    await settle();
+    assert.strictEqual(api.trackChangesPreviewController.isOpen(key), true, "gateway command opens the preview");
+  });
+
+  // SLICE-5: edit controls are inert on a non-authorable (read-only scheme) doc.
+  test("toolbar edit controls are disabled for a non-authorable document (PUC-1/7)", async () => {
+    const SCHEME = "cwf11ro";
+    const provider = new (class implements vscode.TextDocumentContentProvider {
+      onDidChange = undefined;
+      provideTextDocumentContent(): string {
+        return "# Read only\n\nThis markdown doc is not authorable.\n";
+      }
+    })();
+    const reg = vscode.workspace.registerTextDocumentContentProvider(SCHEME, provider);
+    try {
+      const uri = vscode.Uri.parse(`${SCHEME}:/readonly.md`);
+      const doc = await vscode.workspace.openTextDocument(uri);
+      await vscode.languages.setTextDocumentLanguage(doc, "markdown");
+      await vscode.window.showTextDocument(doc);
+      await settle();
+      const api = await getApi();
+      const key = uri.toString();
+      assert.strictEqual(doc.languageId, "markdown", "fixture is markdown");
+      await vscode.commands.executeCommand("cowriting.showTrackChangesPreview");
+      await settle();
+      assert.strictEqual(api.trackChangesPreviewController.isOpen(key), true, "preview opens (reading is always allowed)");
+      assert.strictEqual(
+        api.trackChangesPreviewController.editControlsEnabled(key),
+        false,
+        "Pin + Ask-Claude controls are disabled on a non-authorable doc",
+      );
+    } finally {
+      reg.dispose();
+    }
+  });
 });