refactor(f8): VersionGuard depends on SidecarStore not CoauthorStore

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/versionGuard.ts

@@ -8,12 +8,12 @@
  */
 import * as vscode from "vscode";
 import { SCHEMA_VERSION, isNewerMajor } from "./model";
-import type { CoauthorStore } from "./store";
+import type { SidecarStore } from "./sidecarStore";
 
 export class VersionGuard {
   private readonly warned = new Set<string>();
 
-  constructor(private readonly store: CoauthorStore) {}
+  constructor(private readonly store: SidecarStore) {}
 
   /** True → skip the write path (and warn once per doc). */
   isReadOnly(docPath: string): boolean {