F10 SLICE-1: retire dead attribution toggle (visible/toggle/isVisible + command)

Code-quality follow-up: with editor decorations gone (Task 1), the visibility
toggle was dead/misleading — nothing reads `visible`. Remove the field,
toggle()/isVisible(), the cowriting.toggleAttribution registration, and its
package.json command + palette declarations (spec §6.2 retires it). renderActive/
render stay — they refresh the orphan status bar on editor switch.
E2E toggle assertions (attribution.test.ts, noWorkspace.test.ts) are updated in SLICE-4.

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

src/attributionController.ts

@@ -52,7 +52,6 @@ export class AttributionController implements vscode.Disposable {
   private readonly pending = new PendingEditRegistry();
   private readonly statusItem = vscode.window.createStatusBarItem(vscode.StatusBarAlignment.Right, 90);
   private readonly output = vscode.window.createOutputChannel("Cowriting Attribution");
-  private visible = true;
 
   /**
    * F6 (§6.2/§6.4): the single machine-landing signal. Fired after a real
@@ -70,7 +69,6 @@ export class AttributionController implements vscode.Disposable {
   ) {
     this.disposables.push(this.statusItem, this.output, this.applyEmitter);
     this.disposables.push(
-      vscode.commands.registerCommand("cowriting.toggleAttribution", () => this.toggle()),
       vscode.workspace.onDidChangeTextDocument((e) => this.onDidChange(e)),
       vscode.workspace.onDidSaveTextDocument((d) => this.onDidSave(d)),
       vscode.window.onDidChangeActiveTextEditor(() => this.renderActive()),
@@ -330,11 +328,6 @@ export class AttributionController implements vscode.Disposable {
 
   // ---- PUC-5: rendering ----------------------------------------------------------------
 
-  private toggle(): void {
-    this.visible = !this.visible;
-    this.renderActive();
-  }
-
   private renderActive(): void {
     const editor = vscode.window.activeTextEditor;
     if (editor) this.render(editor.document);
@@ -390,10 +383,6 @@ export class AttributionController implements vscode.Disposable {
       author: s.authorKind === "agent" ? "claude" : "human",
     }));
   }
-  isVisible(): boolean {
-    return this.visible;
-  }
-
   dispose(): void {
     for (const d of this.disposables) d.dispose();
   }